Will He Ever Be Potty-Trained?

My 3 1/2 year old son is driving me crazy!! He refuses to be potty trained. Most days he'll go pee in the big toilet (he barely reaches it). But, other days he's perfectly content peeing in his pull-ups. I know, I know, put regular underwear on him and he won't like the feeling of being wet, right? I tried that and it didn't work. lol
He has no problem with being wet. In fact, he'll walk around in wet underwear for quite a while before he decides to tell me. He refused his musical potty chair. So....I sold it at the last yard sale. The weird thing is that he will pee when we go out somewhere. He loves using other people's toilets. Maybe I should just redecorate our bathroom on a daily basis to make him feel as if he's exploring a new place.
The truth is, I'm at a loss. We've tried letting him pee with Daddy, clapping and cheering for him when he pees, having his best friend tell him that he wears underwear, and rewarding him. Nothing works!
My daughter was completely potty trained by age 3 and I thought that was late. Wow! He'll be 4 in October. I would like for him to begin preschool but one of the stipulations is that he must be completely potty trained before enrollment.
Well...if preschool doesn't happen, I've always got kindergarten, right?