J.V.
They have regressions before big leaps forward. My son was peeing himself and not telling me, and then he started waking dry in the morning. It's the way it works.
No rewards, no pull-ups, keep him in undies and just hang tight. He will be back on track really fast. Also, when he has an accident, make sure it takes lots of time to clean it up, and then explain that going to the bathroom takes less time away from play! And whatever you do, don't make a bath after an accident FUN! We always do a quick, 30 second shower.