A.T.
I would say it is mostly because he is a boy. Boys take longer to fully potty train and they tend to be more busy than girls. They don't always feel when they have to go until they stop and sit down for a minute. My son is 5 and potty trained awesome (at 3.5 years), we never had accidents. Last summer came and he was wetting all the time. So frustrating!
I have a couple thoughts for you. first, do not bring attention to it, this will may make him "rebel" and go more in his pants. Also give him gentle reminders to go potty. Like I said before they don't realize they have to go until they have dribbled a little in their pants. The nap time thing, make him go before he lays down, maybe try limiting his fluids before nap. (I am shocked he is still napping. :) ) I have noticied at night with my son, if he wakes up crying I automatically take him to the bathroom. He is not fully awake but he goes everytime. My girlfriend gave me that tip and it works everytime. The other thing she did for a long time for naps and bedtime, was she would put underwear on first and then a pull-up over top. So the child would feel he wet but the sheets didn't get wet.
I hope this helps. :)
A.