Toilet training a 13 year old, he doesn’t speak

My brother is 13, he doesn’t speak and is still in nappies. He’s getting pretty big so it’s getting really difficult to change him. There hasn’t been much help from his school in that regard and I want to use this time to assist my mum in trying to get him out of nappies once and for all. If anybody has any experience or advice on how to go forward I would really really appreciate it.