Perhaps he is doing it at school - so you don't know if he has done his poo?
The leakage may be an upset stomach?
Perhaps you could let him take ipad to loo with him - instead of taking it away - thus not make it a 'big deal' if he goes or not?
Unless it is really a serious problem such as causing bad stomach ache from holding it in - perhaps it's best just to not make him too worried about it. Not every one is regular!