He's not reaching out to kids in their dms and being attracted to people around the age of 18 isn't wrong, it's just culturally weird. He's not trying to contact these people outside of these public subreddit posts, right? If that's the case, there's no actual evidence that he has any intention of engaging in any sort of predatory behaviour.

People have really knee jerk reactions to these sorts of things because the media makes us hyper-aware of pedophiles and sexual predators. It's all irrational behaviour. Don't judge him for things he hasn't done and doesn't intend to do.

Of course this sort of post is going to get down voted because of people's knee jerk reaction, but I implore you to recognize your immediate reaction and think about if it's actually rational or not.

Also, nobody's said it here, but it's not okay to invade your boyfriend's privacy. It's clearly a private account.