E.M.
Don't ask him if he has to go. Keep track of when he last went and then judge from there when you think he should be ready. Then you tell him it's time to try and go potty. You may want to avoid the whole reward system at home if he is going at day care for the simple fact that he's going to eventually feel as though he should be given rewards for it there as well. If rewards are in order, you may want to consider giving him a sticker chart or something to that effect, but I do suggest avoiding the whole candy issue--even if it is just one M&M. I personally would not postpone training with him because he's in the cast. Yes, you will have to help him get on and off the potty as well as help him with his pants, but you may actually undo any progress that you've made with him so far.
Good luck!