I lurk on there alot actually. My boyfriend think it's stupid and he hates to think about the fact I go on there. Generally in /b/ and /gif/, I got there for humor, random threads, and I guess some times for porn.
I don't see the big deal. 4chan and especially /b/ has a horrid rap sheet for a lot of things actually but it doesn't mean everyone on there is a fat and ignorant prick who masturbates to CP and hacks forums.
Just leave him be. That shows that your not the best friend in the world to him if you think that just because he does something you dislike, that you shouldn't be friends anymore or see him as the person he's always been.