Wanna talk to my son about his kinks but don't know how to

So I've always known my son is sexually active. Wasn't an issue with me because him and his girlfriend use 3 forms of contraceptives (IUD, condoms, spermicide.) Hes 17 and so I try to stay out of his business unless I think he's doing drugs or sneaking around. I wasn't snooping but around midnight yesterday I overheard him talking to one of his friends about his kinks. His girlfriend chokes, slaps him and they like to make degrading comments. Again my issue is not him being sexually active. However I am worried because kinks can be dangerous for impressionable young people. Especially if they're physical. I realize porn is openly accessible to teens and this is where they get a lot of their sexual practices from. I would like to talk to him and just make sure they understand that rough play like that calls for a safeword and 100% full consent. I also want to make sure he's not being hit too hard (and vice versa) because a lot of kinks are honestly just "sexy" abuse. How do I have this talk?