I think first you need to detangle what is your DS's issue and what is the children's issue - what might be the children reacting to DS, what might be DS reacting to the other children. At some point one of the parties is going to behave unreasonably or unexpectedly. Or both.
Once you've identified how the other children aren't being reasonable (or as reasonable as children can be) you can bring that up with the parents. If they say it's your DS's fault then going through the process above will have helped you reason out what is going on and put you in a position to explain it to them.
Arbitrary example. DS wants to play football. Boys say okay as long as you're the goalie, sensing that DS is desperate to play. DS agrees. Boys repeatedly kick the goalie, justifying it that goalies get kicked lots (not a justification but bear with me). DS gets upset and lashes out.
DS's problem there is not understanding or being able to communicate that it's not fair the goalie gets kicked lots or able to negotiate a better deal. He's also not controlling his reaction to their behaviour appropriately
BUT
if they hadn't repeatedly kicked him, then he wouldn't have reliated.
So next time DS wants to play they knew they got away with it last time and do it again, so DS gets upset, so DS doesn't want them to play and intentionally tries to ruin their game so they can't play the game which ends up with him getting kicked in an attempt to play something else.
When you bring it up with the parents they're seeing the end result, that DS is lashing out and hear parts 1, 2 and 4 of the original from their sons and then see DS 'ruining' the game. They need to hear part 3 from you to get the whole story.
IYSWIM. It's not a great example...
I guess what I'm trying to say is be prepared for them to blame you/DS, accept responsibility for what is his part in it (because both sides are probably to blame) but get them to accept their part.
The road back from bullying is tough though Both parties need a clean slate but getting that through to children isn't always the easiest thing in the world.