Ok, so I only have one child and she's 3, so I'm not really in the same boat so I'm really just guessing here, but sounds like you are dong the right thing by not backing down and DH's kinda confusing DS by being the 'nice guy'. IMO DH needs to be on the same page, so DS gets a clear message.

Could you stand by the door of his room to make sure he doesn't come out or have another punishment where you take something he really loves and put it up out of reach but so he can still see it? When he shows a positive behaviour give it back?

Maybe with the big school playgroups you could tell him you will take him to a park afterwards if he behaves. Are there any dangers at the school if he runs away from the group or would it be safe to kind of ignore the fact that he doesn't want to join in and just act like he's the one missing out and tell him everyone else will have a good time without him? Iykwim?

I can see how it would be a hard in such a public situation and big school is huge step isn't it!?

Good luck!