1. He’s embarrassed by them. Everyone has that friend they’re only still friends with because they’ve been friends since birth. It’s their only common denominator at this point because they’ve grown into such different people. Maybe that’s all of his friends, and he knows you wouldn’t be able to stand five minutes in a room with them. Maybe his best friends are a monkey that wears a vest and a magic carpet, and he’s pretending to be a sultan.
2. He acts differently around them. He might think you’re not going to like the way he is when he’s with them. Maybe they bring out the binge-drinking idiot in him, or the giant nerd, or the guy who can only speak in South Park references.
3. They’re dicks. He knows he’d be throwing you to the hyenas like you’re Scar at the end of The Lion King if he brings you around his friends. He’s trying to protect you because he knows they’re super judgmental.