P.M.
I think it's fairly common for a child's sleep patterns to change as they grow up. It's possible your son is simply sleeping more deeply, or even dreaming more vividly and incorporating his pee urges into his dreams (I do that, and am always SO relieved when I wake up dry!). He'll get there again, but these nighttime changes are really not uncommon in the many families I've been close to over the years.
Since sleep is so important (his and yours both), my inclination would be to keep him in overnight diapers until his body and brain catch up with each other. There's really no way most of us can make ourselves wake up at night, though I have heard that if you actually encourage extra drinking before bed, the full-bladder signal might become strong enough to wake the child.
Good luck – I hope this spell passes quickly. A good rule of thumb is to go to underwear when the child has had a 2-week accident-free period.