Hi L.,
At 3 1/2 my son had absoultely no interest in using the potty. I could tell that he understood the process, but just wasn't interested. Taking his diapers or pull-ups away would have done nothing except give me a mess to clean up. Being in wet or messy pants did not bother him at all. Smelling bad did not bother him. And he has never been embarrased about anything for a second of his life. He does not has a strong desire to impress or please other people, me included. I thought it was hopeless.
Everyone told me to put him on a potty every 15 minutes. We tried it a couple of times. He would go occasionally, but he still had accidents and it was very labor intensive. It was not something that I could do long term.
One day it just occured to me what I needed to do. My son is very independent. He always wants to do things himself. So, I let him pick out a new potty. (the big baby bjorn is good for bigger kids) I put this new potty in the room where he spends most of his time. Then, I showed it to him and said, "From now on, when you need to pee or poop, you should do it in here." I gave him the control of the situation. That was the key with him. He needed control. Making him sit on the potty every 15 minutes was taking the control from him. You need to figure out your son's key. I think taking away pull-ups or sitting on the potty every 15 minutes works with most kids, but not all. Yours might be one of those that needs something different.
When I showed my son the new potty, and gave him control of using it, he immediately took off his pants and diper and went in the potty. And he has been using the potty ever since. He want naked at home for a couple of weeks. And he did have a few accidents. But, just a few. Once I figured out what he needed as an individual person and quit worring about how all of the other kids get trained, it was really like magic.