Nawty...
OK:
Then after being told he was in remission and would pull through, I realized he was milking the illness to get things done his way. I put a stop to it and that is when all he!! broke loose. It has made him a stronger (mentally) kid, but he is different from other kids. He would rather hang out with adults (teachers, my friends, kids' parents) than other kids.
You're there so, consider this merely a complete strangers observation from afar. You gotta figure it out and handle it from there.
Kids (and I suppose adults as well) that go through a really big deal like he's been through will, like everything else, handle things a bit differently from case to case. He is decidely NOT the sterotypical kid who just wants to be treated normally and get to go swing and climb the monkey bars with their peers.
You say he's stronger mentally and wants to hang with adults. I'll wager he thought his doctors and nurses were cool as hell and was interested in them as oppossed to being afraid of them.
I think you're stuck. He ain't no 6 year old up stairs and having to behave like and be around real six year olds is tourture to him. They are all there to become reasonably conforming members of society and your boy already knows there are far more important things than school. You showed him that.
Frankly, I don't see how you couldn't, so let that go. I know damn well if I was facing losing a kid at any time, behavior and rule following would be out the window to. We'd be eating ice cream for breakfast, staying up late and taking movies back two days late.
So, he's older mentally. So explain it.
"Jr, before, I thought I might lose you so, we treated every day like it might be the last. The good news is you're well now. The bad news is you are now expected to live a long and productive life which means we are no longer treating every day like it's the last. Which is actaully good news as well though you don't think so right now. So, you have to go to school and hate ever last second of it, save recess and lunch, just like all the rest of the boys and you have to act like it's OK. So, grace period is about over and if you don't get a clue we're going to have lunch with Auntie Vraiblonde and she will remove all pretext of trying to be nice about all this and and kick your ass up around your hat, so to speak. Either way, you don't get to act like your an adult until you actually become one so, glad you're feeling better, head or gut?"
That'll do it. He just needs to get with the bigger picture, just like he did with cancer.
Sounds to me like he can handle it.