You haven't said what all you have tried so it's hard not to tell you something you may have already done. But my daughter was a stubborn potty trainer so by time she was coming up on 3yrs I just started putting her in underware at home. If she had an accident, she had to clean herself up and toss her dirty clothes into the washer (she didn't have to wash them). If we left the house I would put a pull up on over her underware, and same thing, if she peed while we were out, she had to clean herslef up when we got home. After about a week of that, she tosed in the towel and started to pee on the potty. She would have an accident here and there but it was all good.
You can also try potty candy, potty chart with stickers, etc. For your son, make sure he is sitting down and not having to stand unless you like to wash your toilet, walls and floor every day! And you can also tell them that they can't go back to prescshool if they don't pee in the potty as well. Most kids love going to school so that might be the push they need as well.
Good luck!
S.