Ds3 took a while to persuade out of nappies, but was 'clean' during the day by the age of 3.5, although he is still wet at night. Recently, he has been coming home from school with dirty pants, not a lot, just as though he hasn't managed to get to the toilet in time, and a bit has come out too soon (sorry, TMI!).
Then last week, there was more mess, and he actually smelt bad when he got home, so I started to make sure he'd done a poo before he went to school. This hasn't helped- he's still coming home dirty 
. He says he doesn't know he's doing it, that it just comes out without him knowing it. He says he doesn't have tummy ache, and he goes to the toilet when he wants to at school (and obviously at home, too).
Today, he has lost control twice, and made a big mess in his pants this morning, and just now, in his nappy, about five minutes after getting into bed (after going to the toilet before the nappy went on).
The fact that we can't get him dry at night is concerning enough (although, after reading threads on here, I'm trying not to make a big deal of it, and planning to spend the summer holidays making sure he drinks loads during the day, in the hope that he can stretch his bladder and train it to hopd enough to stay dry at night!), but this is a real worry.
He has no special needs, although his big brother has ASD, and he is not obviously poorly. Is there any explanation for this sudden regression/loss of control? Should I be taking him to the GP, or getting strict and monitoring toilet visits?