Label the behaviour not the child. The behaviour was nasty. Some of the time the children sound as though they were being okay, some of it not. 8 year olds are very, very rarely nasty in themselves.
It sounds like they are fairly unsupervised/ left to it, albeit with parents physically in the vicinity. From what you say they were using your DS as the easy option to be 'It' because its the role nobody else wanted.
With Autism our DC are more vulnerable to being sidelined as they don't pick up on the nuances of play in the same way but many can learn to challenge unfair and assert themselves to redress the balance or learn when not to play.
My own Autistic son is now 18, so I have been the hands on parent feeling very defensive/ protective/ angry for my child/ frustrated at other lazy parenting.
Have you come across social stories and story boarding? Basically breaking down a situation where things haven't quite worked out in an ideal way and looking at the way they could be managed encouraging your child to tell the story and work through different scenarios.
So in the 'It' scenario.
Was it fair DS was always 'It'.
What were DS's choices. Say nothing and accept being It. Ask that the 'it' role rotate or walk away saying I no longer wish to join in if we aren't sharing.
If you go through the story with DS of what happened a few times and find yourself in a similar scenario, you can mention the story of the park and encourage your DS to consider his options. Let him lead the decission in how to act and positively reward him owning the situation when he makes a good choice.
You can use the basic sharing scenario to make up other situation stories and talk through with your DS what choices would be.