bro sometimes people have healthy cross generation relationships either with older relatives or neighbors, could easily mean drinking or going to bars or shit that a 20 year old would think would be fun. not always about sex.
Dude he was actively trying to be in a relationship with him
hence why i said "sometimes people", meaning im addressing bout the general statement in your comment.
and it's been a while I've read this but iirc 2nd ML didn't do shit like keep in touch or actively woo the MC before they got reunited
it could be he also had a crush (which doesn't have to be malicious just bc of the age gap btw, maybe like "oh wow i sure hope i can find a partner as cute as him" kind of thinking). in any case i was expecting he had no interest when he first showed up and only actively became a hindrance to introduce conflict to the story.
Mc was in middle school while the 2nd love interest was in college and literally every other convo they had he was saying things like "when your 20 we'll do this" and "when you become an adult we'll do that"