Can he get out of his bed or is it a cot still? Try him in a bed with a potty in his room, explaining that "this is where we wee".

I know my DS can undress himself and take his nappy off, thankfully he's not done both at the same time yet, but can you get a large plastic sheet for his matress? We got a wee-proof matress for DS because we're anticipating this problem.