What Olive said.

But if he is showing signs of readiness could you could teach him a sign (as in sign language), so say "toilet" and use the sign each time.
Otherwise, be positive, lots of over the top celebration if he does something.
Whatever you do, don't punish him for accidents, it really, really doesn't help. Just say "oops, oh well, we'll try again next time" in a happy voice.