It can be tough when kids don't seem to be having ... positive interactions. My DS had an older friend when he was 5 (10, now) - and I thought that kid was just rude. But DS didn't seem to notice, and still wanted to play with that little creeper. Boys are odd. What does your DS say about the boys? Could ask him in general ... How was play today? If he says, I had fun! then ...boys are odd. If he says, "So-n-so was mean to me ... or wouldn't let me play." Well, that would be different. You could try to explain it, maybe? "So-n-so is a good friend, but when he's with the older boys, he's just going to act different."
I understand what you are feeling, because when that kid was being rude to my DS, I really wanted to tell him off. That would have been ... some comedian I watched had a bit about this kid who was a jerk to his kid, and he couldn't stop thinking about what he should have said to him ... That was me! So, try not to let the behavior get under your skin, I guess.
Finally, I spent a lot of time with my son,going to parks, zoos,aquariums, etc., so we had a lot of positive interactions, and I think that helped to play down any not so positive interactions with other kids.
I wish I could tell you how to get those boys to behave! But, I can't think of anything, except maybe indirectly bribe them? Sometimes if kids really like the parent, they'll be nicer to the kid?
Best wishes!