If you are trying to gain weight for yourself and not your boyfriend, a good ice cream like Haagen-Dazs before bed can help. Also cheese, potatoes loaded with butter/sour cream, beef, coconut, etc. Zinc is needed for appetite regulation, and you're probably deficient; if you order liquid zinc sulfate, hold a shotglass of it in your mouth and taste nothing, you need to supplement. I'd also suggest avoiding all gluten, even though it's tough; it can cause gut irritation and malabsorption, making it difficult to put on weight.

Good luck with the boyfriend; the others have left good advice. Sounds like a shit situation to be in.