3 year old weeing himself

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My 3 year old has started completely weeing himself. He is fully toilet trained and has been for over a year. I don't understand it! He doesn't seem bothered that he's doing it either! Advice?

Is there anything that could be troubling him at the moment? A change in his routine, I.e new baby, house move. Has he been poorly at all? It's hard to say without knowing a bit more but hopefully someone with experience will be along soon. Good luck!