I wish I had a good answer for you, it looks like you tried everything I did - same situation, except we were sometimes yes, sometimes no on the pee-pee in the toilet, he would just forget, and he has pooped a total of 3 times in the toilet, ever, he'll be 4 in April. He didn't want to fall in the toilet either, so I put him on it backwards, which helped for peeing, but not pooping, then I introduced the little potty and I made him sit there. It was a painful, agonizing first 15 minutes. I knew he needed to go, he knew he needed to, but he just wanted to do it in the corner somewhere. Luckily my neighbor was over, my son loves her, she's like a grandmother to him, and she knew what to do. She sat next to him and read him books and just tried to distract him and encouraged him when he thought about it. That was back in the fall. He has gone only 2 more times since then, both times big struggles to keep him on the potty, one time I litterally held him on and told him he wasn't allowed to get off until it came out, he cried, then pooped and was so happy about it, that he forgot that I was mean to him.
Everything changed for my son this week! Monday night we put a clean pull up on for bed and my son has kept it dry and clean even until now. We're on the same pull up! He has never made it through the whole day in the same one. So, here's my advice, be persistent, consistant, and patient. He'll do it when he is ready... Best of luck.