I don't think the problem is you. Sometimes these things just happen. It doesn't mean he doesn't like you or you're not desirable; biology is just weird. Do you think it's possible he's having performance anxiety? Are there other stresses in his life, school, work, etc.? My guess would be there's some other stress that's making it difficult for him to stay erect during intercourse. I could be wrong but I think that's a place to start.