Sounds like you're pushing it a bit much. Our 5 year old started doing the same things as your son, and we got the potty chairs so they'd be there when she was ready. We basically just waited until she was closer to 3.
There are a couple of things we found that worked, but it was a process. At the sitter's she liked using stickers, but that never really worked for us.
The thing that did work pretty well was setting the timer for every 30 minutes, and letting her on the potty whether she had to go or not. Also, tying in the "potty first" rule. Anytime she wanted to do something different (go watch a show, or color, or play with toys, get a drink, have a snack, etc,), she had to go potty first. It was pretty much, "You can do whatever you want or have whatever you want, as long as you go potty first."
There will be times after he's "potty trained" that he just will not want to take the time to go potty and will go in his pants even when he "knows better." Then we'll just start setting the timer again, and making everything "potty first". She knows that if she takes time to do it herself, then we'll back off. I'm sure that sometimes she does it for attention, but it saves me from doing a mountain of laumdry.
Good luck!