Same issue here w/ boys I watch...3 1/2 yrs old w/o being potty trained - and they are VERY smart. I took away their pull-ups. As soon as they started peeing on themselves and getting all wet and THEY had to go change their clothes they started using the potty...the pooping took a couple months but they also have that down now too. I just stopped changing their pull-ups, their clothes, and handed pottying over to them.
I'm in the same boat. I'm letting it be his choice. If he's 3 and still not trained, then we'll get creative. For now, it's nothing but a power struggle, and it's not worth it. He's not going to do it until he's ready anyway.