Advice Needed: My 3 12 year old son is having a hard...

You've got to get rid of the pull-ups and just have some Resolve ready.  You are confusing him by putting him back in pull-ups to poop.  I'm sure he knows he has to poop, sometimes kids are scared of doing it in the potty.  Some kids feel like a piece of their body is falling away.  Keep him in underwear and when he has an accident or you see him showing signs of pooping, rush him to the potty.  Don't fight him over it, he'll win.  Just keep encouraging and supporting with tons and tons of praise.  Have him help clean up the poop accidents and while you are cleaning, just reinforce that poop goes in the potty.

Good luck!

I would not do the pull ups. Get rid of them. Have him help clean up his poopy and remind him that the poop goes in the potty.

U definitely need to get rid of the pull ups. I went through this with my daughter who is 4 1/2. I still have an issue with the pooping. She has this thing where if we are outside she has no issue pooping outside while squatting but on the toilet she can't go. I'm having her checked out to see if maybe it has something to do with her hips. But definitely get rid of the pull ups. Most of my daughter's underwear is stained but it's worth doing the extra laundry as long as my daughter no longer goes in her pants (first 3 months after switching to just undies she was pooping in them and now she's pooping outside lol or on the toilet)