K.S.
It is possible that he is relieving himself enough before bedtime, but I would still think there would be some in the diaper. We are also going through the potty training and after naps, he is usually dry but not in the morning.
If you are concerned, increase his fluids. Does he drink milk or water with dinner? Ours has water with his meal, and a cup of milk right afterwards.
If he is still going pee through out the day on the potty or in the diaper, don't worry too much.
Does he potty right when he wakes up in the morning? If so, then he is simply holding it.
Increase his fluids and if that does not improve, I would call your dr. Dehydration can be scary.