what do you do when 3 1/2 year old...

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poos minute you close door at night in his nappy

before did call us to go to potty or to change him

but now even after telling him off (not for pooing but for not calling) and removing stars etc he is now not even calling when dirty

l;iterally poos minutes after we close door and after has been sitting on potty for 30 min

been going on for months

at end of tether

Oh poor you, how frustrating, have you tried leaving him in pants, and not putting a night nappy on him. Or just pajama bottoms. He is clearly just waiting for the nappy. You could use bed mats to stop matteress getting wet if you left him in pants instead.

Does he do poos any other time during the day, or does he just wait till bed time?

I had an incredibly similar experience with my DD. Is your child dry with weeing?
I suspect mine wasn't quite ready and had a few privacy issues! The turning point for us was on holiday when we got out of synch with nighttime toileting and DD had to go during daytime and I refused to put a nappy on her - she got so desperate she had to use the potty and from that day has been clean.
My DD, like yours I suspect, had come to associate the nappy going on with having a poo. I guess we were lucky that the holiday broke the routine.
DD had made some steps towards being clean - she got the idea that poos went in pottys, but got around this by having the poo in the nappy - removing the nappy and emptying it in potty and then putting the nappy back on
Not nice!

Could you try putting him in a nappy half an hour before bedtime so he can poo in it then if that is what he is waiting for? Then you could change his nappy before he goes to bed.