lol! no worries
As far as not socializing with other people, IEIs can get really shy. There have been quite a few times where I have wanted to socialize with people but was too inhibited to, despite wanting to do so.
I start to think 'ok I have to talk, do it now..no, not the right time..now, aww crap should have done it then..now its weird, omg its getting weirder say something' etc etc until it snowballs and Im paralyzed with anxiety. So, it could be that hes actually hyper aware of whats going on and its getting to him(we can get pretty self conscious at times, lol).
I read on wikisocion that one of the roles for IEI is good natured tag along, and I feel like I can get stuck into that role if not open enough. Sometimes, all we need is a little introduction or a little interest shown our way for us to light up. We can feel cut out from the group sometimes, like outsiders looking in - from this vantage point it becomes very hard to socialize. We definitely dont want to make a bad impression, I dont know if other IEIs are like this, but I feel that its better to stay quiet sometimes than say something awkward - which I am more prone to do in that state of anxiety.
It helps to know that its ok to be a douche, like its ok to fuck up. People telling me to relax and go fuck up and have fun help; but also people making fun of each other and ME lightheartedly in a group helps me open up and not take things so seriously
As far as the road trips, ahhh! that must suck. Seems to be a common trait among IEIs to complain excessively. Introducing him to some philosophy and self improvement principles/books/programs may help tremendously. It has a lot of useful mindset shifts that would help him have a better outlook if implemented properly. If hes an IEI, he would probably be into that sort of thing.
My advice though is to talk to him, tell him how its affecting people emotionally in a really non judgmental way. There could just be really simple misunderstandings going on. I know you just care about him and want whats best for him, he may genuinely be unaware of how its affecting others. Don't attempt to force him into anything, but attempt to compromise - LSI have always been reasonable people to me so I don't think this will be too hard a feat to accomplish. With the right attitude and a little persistence, we're more than likely to meet you halfway.