L.W.
I don't know that I would be too worried but I would (PRIVATELY) mention it to his pediatrician (maybe by phone) and see if s/he wants to see him to check any medical issues. Does your son have any sensory issues? Here is a link with information:
http://www.childhoodanxietynetwork.org/htdocs/htm/sid.htm
If yes, then it is part of sensory issues. My son is now 7. He still occasionally wets (not all the way usually) first thing in the AM or middle of the night. Boys are busy playing and don't pay as much attention.
I just tell my son it is time to go potty throughout the day, even now at age 7.
Best of luck! Don't worry too much though :)
L.