S.D.
My 4yr old does the same thing - he's been potty trained since about 2 1/2yr old or so. Drives us crazy. We tell him to go throughout the day. Somedays he has no accidents, sometimes 2 or 3. Same as your son, just enough to soak through the his shorts. And he often fights us when we tell him to go - "but I don't HAVE to go potty!", then he goes like he's been holding it in forever! We tried being calm about it and having him change, just hasn't worked for us. We actually have started with if he tells us, it's ok, he just has to change, but he hasnt' done that yet. So usually what we do if we notice he's wet is ask him if he had an accident or pee-peed in his underwear - we give him 2 chances to tell the truth, then take something away (we start with the TV, then move onto a favorite toy if he does it again). We take the thing away for the rest of the day and he gets it back the next day. It's a mix of discipline and getting him to tell the truth. It's obvious he's had an accident, but he needs to learn to tell us when he does it and tell us the truth when we ask him (cause he doesn't tell us). And we keep reminding him that if he tells us he had an accident, nothing gets taken away and all he has to do is change his pants then go about the day. It is definitely the age. Like I said, somedays he does great with no accidents, others he can have up to 3. Good luck and just remind yourself that "this too shall pass" ;)