The little boy I watch doesn’t wash his hands!

Help!!! So I’ve been watching my neighbors son who is 8 years old. I’ve noticed he doesn’t wash his hands after using the restroom. I told him to wash them and he got pissed off. I’ve spoken to his mom multiple times and she assures me he washes his hands at home all the time. What can I do to get him to wash his hands when I’m watching him.

*I’ve tried standing there, he won’t wash them

*I’ve tried hand sanitizer, he won’t use it

He swears he washed them but i know he doesn’t.