dweia, i sympathise! my son is a lot older than yours and still unreliable; he would not go to sleepovers for years.
he also has been found in his bedroom weeing in the middle of the night...down the wall once, from a top bunk. he does sleep very deeply (also talks in his sleep, i don't know if that's relevant.) it isn't a psychological problem in his case either.
anyway, i didn't come on to depress you, just to give you some options! first of all you can try to increase his functional bladder capacity, so that he can go longer between wees; you'll only be able to do it while he's at home of course. you have to increase his drinking during the day and encourage him to hold on as long as possible before he goes...easier said than done of course. (you're trying to train his bladder to go through the night.)
you can also try the buzzer alarms but, again, with a child that sleeps really deeply they might not work; when we had one of the big box alarms we put it inside a biscuit tin and it woke the whole house but not his lordship whose head it was next to!
we have tried desmopressin tablets but they don't always work; actually i have just read lou33's message, i didn't know there were alternatives.
what we are realising now is that being overtired makes it worse. we have battles to get him to bed at a sensible time but when we do it makes a difference.
he is still not reliable but goes to sleepovers with a sleeping bag with a huggies pullup (giant size!) concealed inside and, amazingly, seems to be able to get away with it.
i can't think of anything else just now but good luck!