Hey C.! This is a tough one! I have a 3 1/2 year-old son also, and he just recently potty trained. Partially taking the advice of a friend's pediatrician, I used a little bit of reverse psychology. I told my son that he could be a baby as long as he'd like. BUT, he would have to GIVE ME one of his big boy toys every time he pooped or peed in his pants like a baby. For my son, it was his Thomas trains. One at a time, I eventually almost earned all of his trains. Sometimes he wouldn't willingly give me a toy....so, I took it myself and showed him what I took. I did not give him back any toys for going on the potty. I told him that after he was a big boy going on the potty for a while (for him, I gave it 5 days straight of dry/clean days), I would give ALL of the toys back to him PLUS a new toy of his choice. In the mean time, I gave him gummy bears for pooping and peeing on the potty. I just kept telling him that when he wanted to be a big boy he would get his big boy toys back. And, I said, it didn't matter to me...he could stay a baby as long as he wanted to. I was just going to be having fun with his big boy toys in the meantime. Also, I kept the toys I took away in plain view where he could see them (above the refrigerator).....which was easy since they were small toys.
This process probably took me about 6 weeks. Good luck with your little challenge!
A.