I wouldn't say I like the shota genre, but I do read shota occasionally. Mainly because I tend to focus on things I want/like in manga and ignore things that would bother me (like the character's age making me want to call the cops if irl).
I also don't really worry about ethics or morality when reading fiction - I've watched a couple Fast&Furious...... reply
Tell him he doesn't have to like the same things you do, and if it makes him uncomfortable you won't talk about it with him anymore, but that he can't call himself your friend and then say he'll cut your relationship for watching a genre he doesn't like.
If he still insists on it, I can only see it as him being extremely homophobic, or having never...... reply