My boyfrien's friends

My boyfriend tells me he downy drink or smoke and I believe him 150%. But his best friend do it and he hands aroud with them when they do. I don't understand how he can be around people that do things like that. (All under age) I trust him, but it hurts me to see the way his friends act and how few tolerates such disgusting things. How do I deal with this. I can't tell him to stop hanging out with him, but I can't feel safe knowing he does hang out with them. I'm so torn