(pretend you don't know I've already made a thread re: him and approach with an open mind. i think my description this time is a lot more truthful and fitting, less emotionally based and more detached. as detached as i can be. a lot more time spent with him everyday since school started has improved relation so much, so i think i've gotten to know a lot more now.)
** sorry, i merged some questions re: enneagram stackings and such here too **
He is very dull and quiet on first impression, as if he's really deadish [xx(] Not exciting, etc.
Very bad with making small talk with new people. Around them, he usually will just ignore them and focus on the person/people he does know. This gives off the impression that he's not too friendly.
Dresses well, knows how to take care of himself without difficulties. Makes it look easy. Seems very stable, solid. Helps me with my lack of sp, when I forget things (all the time), he consistently remembers to bring them for me. 
Him and I get along very well together when I become very expressive, show off my emotions so that there is no doubt/questioning left. If I don't exude my feelings happily all the time, it's hard for him to create that emotional atmosphere. He responds extremely well to big smiles, lots of joking/teasing/playfulness. (This is what makes me think he is more of an INTj than an INTp who would just be annoyed with all my Fe, for those that know what I'm going on about!)
Has said that he finds himself "mirroring" other peoples - I think he means other peoples' general mood, or actions. If I am dull, he will be dull, if I get very lit up and excited, his ice-cold barrier crumbles very easily. [:I]
Really loves to read, constantly does so. He is limited to the fantasy genre though - enjoys all that crazy stuff about elves, dragons, etc.  Loves anime as well. In general just seems to enjoy that sort of fantastical escapism type of story.
Is definitely an introvert. Will not be easy to converse with regarding personal matters unless you are special to him/very comfortable on both sides. Once he is comfortable though, he can really go on about very random things. I've noticed more of an inclination towards telling personal sentiments/stories with many undertones of humor. Doesn't seem to have a problem with being serious or philosophical, but can be really light and fun too.
Has said he doesn't usually have long, drawn-out conversations with people. He almost made it sound like that was foreign to him, as all his conversations were more business-like and stayed directly on the topic rather than straying/digressing in so many different directions as it always does when WE talk. He said he doesn't talk like that with anyone but me. [)] It surprised me cause he's quite a good conversationalist when in the mood.
As for his stacking, sometimes I really can't tell. He seems quite adventurous actually, underneath the dull exterior (that's only in the beginning) ... then I started finding out about all kinds of strange shit he has done, such as pierce his own labret, have his first time driving while he was totally high on a lot of weed... things like this.
Doesn't seem to have a problem with craziness such as the above and actually embraces these things when the opportunity arises. He will always act upon his ideas or mine, disregarding danger (unless it is really dangerous, obv.) Is actually quite positive and reassuring when I have doubts and such. I am usually the one more fearful to do certain things.
Has said that he would be the type of person who would love to stay out all night, go to raves, in general act crazy, were it not for his parents holding him back. This points against sp first, does it not? The "real" sp-firsts I know always tend to really enjoy their solitary time, while he (despite being an introvert) really wants to do things, get away from the boredom of his house.
Has something of a weird relationship with food. Is never hungry apparently, so he says ... usually just drinks a lot of coffee and runs on that the entire day + 1 actual meal. Parents and doctor thought he was starving himself or even anorexic but he assures me he just does not feel the need to eat. Somehow I think there is more to the story than that.
Likes to join extracurricular activities/clubs so that he won't have to go home so early... seems to value the idea of being in a club.
On the other hand, he has social anxiety of sorts. Mentioned that he is sometimes worried in new groups of people whether they will judge him for saying something stupid, brand him a loser, nerd, etc. Cares for the opinions of other people.
Seems to have a problem with talking/addressing authority unless totally necessary - he will avoid it. It is usually me who has to go up to them to ask a question, etc. He will also just be bad with strangers, in general so I balance that part out well, as I seem to lightly touch base with everyone around me (my so instinct)
Definitely seems to focus on the individual more than the people around him. I can totally see wanting to merge/bond with another person entirely more than being part of a social group. (sx instinct)
Values relationships well. Doesn't just jump into the physical stuff as other guys tend to do. Takes a lot of time and effort into getting to know someone properly before feeling it is "right" to do anything physical with them. This may have stemmed from a bad experience with an ex-gf with whom he was too physical with, and lacking emotional/mental connection.
After that bad experience, he says he would rather keep relationships secret because of rumors spreading - another fear regarding social groups of people. This is how he and ex-gf broke up - rumors, so he is wary to let that happen again.
Seems to like figuring out the whys and wherefores of how things work. Very logical deep down. Is pretty good with figuring out practical ways of making things work, fixing things - likes mechanics. Doesn't love it, but is just natural with that sort of stuff (unlike me!)
Doesn't seem to place much emphasis on the future - is very unsure of his own personal agenda regarding it. Doesn't know yet what he wants to do with his life after graduation from high school.
Wants to be a doctor (that is what he is most interested in, but still not passionate like me regarding psychology) - knows a lot of facts about biology. Enjoys physics too.
Likes playing random puzzle games (pen & paper) that exercise the brain - not sure if this is Te or Ti related. Likes chess a lot too because it offers him the chance to figure out anothers' gameplay/strategy which he finds interesting to do. Is chess liked more by LII or ILI?
I am definitely sure he responds to Fe now - I had an experiment one day because I kept holding myself back for the longest time and things were becoming very strained and unnatural. I decided to myself that I wouldn't question anything anymore and just be myself - that is, a lot of Fe! It worked like a charm and after that, things ran so smoothly where we could both just be ourselves. I don't feel held back at all anymore, I am as expressive and loud as I wish to be and he responds to that. Sometimes I hate socionics! It prevents me from being myself because I thought for a while that he would revolt from my Fe, therefore not being myself ... am learning not to take everything here so seriously anymore! It definitely hinders a relationship.
In general, the relationship between us is very good now ever since I started being myself and I've noticed that he gets on very well with Fe people too, my IEI buddy and some others who I have not typed but seem to be SLEs or ILEs. Something within the Ti-Fe axis, at least.
However, he tends to believe in everything he reads which irks me at times. He will say some random fact about something he learned regarding a subject or something which might make me go : abuh? That makes no sense, where did you read that, why do you believe it, etc. This is probably my rejection of Te.
Confusing, eh?