W.M.
I had the same problem with my son--he was that age when we started seriously potty training too. First off, I learned not to ask if he needed to go--he would never say yes and then have an accident. I set the timer on my phone (I let him pick the tone/alarm sound) for 30 minutes at first and then upped it every day or so. When the timer went off, he knew it was time to go potty--at least try. Typically he would go and if he didn't, I set the timer for 5 more minutes and we would try again. It probably only took a couple days before he had the routine down. As he got better and more comfortable, I could ask him if he needed to go and typically tell if he was lying (dancing around, holding himself). And no pull ups (except we still need them at night), those just made him feel comfortable enough to have an accident. We probably only had 5 accidents the whole time we were potty training.
Also, ask the sitter what she did to make him go all day and try to both be doing the same thing, whatever it is that works best for him.
We did do the rewards thing for my son...he got a sticker and an M&M every time he pottied. When the sticker chart was full, he could get a cheap DVD ($5) or hot wheel car or something. We tapered that off once he had the peeing down and only used it when he pooped in the potty (that took longer).
Hope this helps--good luck. It is super nice having him potty trained and if he's ready and willing, then it should be easy. If it is stressful on either of you, it won't work--I've seen it firsthand with close friends and their kids.