Tell him that you WILL nag him every month and will want PROOF that he's paid the rent. If he doesn't agree, insist that he put you on the bank account, if you're not already and you will make sure the rent / bills are paid. Take his bank card so he can't buy himself anything daft and give him some money for travel / lunches and stuff.
If he wants to behave like a child, treat him like one. Believe me, being threatened with that changes your behaviour very quickly.