The nappy
as you wash the doll with him (or leave it completely up to him to do), spell out the words well and repeat each action in your voice. This also applies when you are in the bathroom to wash your little hands: e.g. “turn up the tap lever, hot water comes out, get some soap, wash your little hands now, rub rub rub”. This way she will memorize the sequence and it will also help with her language skills.
A funny potty
Buy a fun potty and if at first he pees outside the potty don’t get mad, accidents happen, just let him know that if he needs to pee again he will have to sit on the fun potty he just bought! Before taking it off during the night, start with the day; then put a waterproof sheet on the mattress when you start to take it off at night. Then read with him one of the suggested books on this very subject!