Now what do I do?
Stop sending him.
Honestly it is that easy.
Your job is to protect your child. If they are not looking after him properly, then dont send him.
When you go with him and she gets him incorrect or not enough food, take it off him and get him something else or supervise and intervene.
YOU are his parent. If your mum doesnt like they way you do things then thats too bad.
They dont have a RIGHT to him.
You tell them that he wont be having sleepovers any longer as they arent listening or even remotely doing what you want. You will be more than happy to bring him for visits, or they can come to you but other than that, sleepovers arent happening.
If you are sending him due to work then try other baby sitters.