IIN that I'm 18 and not interested in partying or going clubbing at all

perfectly normal, I've never enjoyed the party scene, and most my friends were the same.

growing out of it is normal, in college I had a friend she was a HARDCORE party girl, like she got word slurring drunk, and then she picked the guys up in bars to fuck regularly. Hardcore party girl, but in this last 12-18 moths she's been leaving the party scene, she doesn't drink much anymore and she's been trying to settle down.

As for what is fun to do depends on his interests, I'm a nerd and a kid at heart, so board games or acting like an idiot playing on playground equipment at night would be fun. If he likes sports take him to a game or plan a tv game viewing party at home.
But whatever his interests I promise nothing would make him happier than you sucking his dick all day, no matter our personal interests all guys have one universal interest.