Have you tried a reward chart? Every time he cooperates, a sticker goes on, leading to some big treat at the end.
Or just keep taking a toy or favourite book away every time he does the curly up thing. He might not mind at first, but maybe having toys systematically removed and not returned until he starts cooperating might get through to him.
And explain to him that his teacher will be keeping note of the number of times he does it at school too, and the corresponding number of toys will be taken away when he gets home.
Just suggestions really, it's a tough one. Hope you crack it.
Dd2 is 3.3 and has never liked being told off. The sound of a raised voice has her in hysterics it always has. She will not do time out and generally ITS hard to reason with her. No advice except to say am in sort of same boat
Should add this is escalating because on Friday I had a call from his teacher saying he had curled into a ball after she tried telling him off & she didn't know what to do with him. This is a teacher with over 20 years experience. I spoke to him on the phone and he got up and behaved, but the teacher is not happy.
Ds2 is nearly 4 and is starting to be a problem at school and his new after school. The teacher/childminder asks him to do something and he basically curls into a ball and refuses to do anything. It's a brilliant tactic. At home we react by excluding him until he comes out because as he goes limp there isn't much more e can do. Part of the problem I think is that it's difficult to motivate him. Threats don't work. He doesn't care if he has to go to his room, he doesn't care if his toy is taken away, he doesn't even care if you threaten to smack him. We do follow through, threats are not just idle but he just doesn't mind. He is largely a lovely boy. Smart, learning to read, counts to 20 etc. but I think he just likes to push the boundaries to see how much he can get away with.