Help please!!

My son is 4 next month. He has been properly toilet trained, he knows when he should go to the toilet and he knows how. He is also fully night toilet trained and has only ever wet the bed 3 times since being toilet trained. (which was about two years ago).

So he is wetting himself during the day, to me it seems out of laziness, as he doesn't even have to be distracted to not make it to the toilet. He will not tell me he has wet himself, I find out when I start to smell it. He has even had multiple rashes from sitting in wet undies, still this does not deter him from doing it.

I have tried everything from, being cool, calm and collected and talking to him about it. To strategies, like saying he has to wear a nappy again, getting angry, making him hand wash his own wet undies and pants.

Nothing seems to be working and I have just about had it, I can't seem to stop it from happening. He has no urine infection and is a perfectly happy healthy child.

I have even tried telling him that he starts big school next year and he can't be wetting himself at big school.

Please help me, I am stuck on what to do or try next.