Here's an idea from my pediatrician that I hadn't thought of. My son was three in July and at his well-check appointment the dr. and I talked about potty training. I had actually just started (variety of reasons...) and it was slow going. Anyway, he said after a child is age three, it is completely in their control biologically. He suggested a reminder every hour or so....look at face and say..."it's time to try the potty" (don't phrase it as a question)...if he doesn't have to go, then say "ok, but if you have an accident, I'm going to take away that tractor (whatever he's playing) for a little while." Then if he continues to play and has an accident, put the toy out of his reach but where he can see it (we use a bookshelf). Tell him he can get it back next time he goes in the potty. So, this really stepped it up for my son. He went from accidents about half the time to only one or none a day. The key of course, is taking away something that he really likes and wants back. You may even consider taking that "big reward" that he earned initially. Also, it doesn't put a reward for being accident free, but does allow for a consequence for having an accident.
I like the idea of the potty watch that another person posted. You could maybe give him that...and explain that it is reminding you that it might be time...but give him the control to choose if it's time or not. Then take away things if accidents happen.
Another thought...is your other boy ready to start? It might help the older one to be the "expert" in teaching his younger brother. We rewarded both kids with a jelly bean every time the little one went successfully, because the older one helped with the reminders, running to the bathroom, reading books on the potty, etc.