I send your son and leave them to it.
There are a lot of women who don't think that their exes or exMILs are capable of looking after them. If you think there is serious neglect going on, pursue a court order. Otherwise, send your child to his other parent, ill or otherwise, and let them deal with it. He's never going to know how to look after him when ill if you don't let him.
My exh doesn't look after our children in the way that I would, I don't do things the way he would. I think he's wrong, he thinks I am. I disagree and so does he.
But I'm not the boss or the owner of the children.
Maybe with a bit of practice he will not to do.
It is irritating when mothers (and it's usually mothers) hold their child's other parent to exacting standards, insisting it's their way or no way, in order to limit/restrict contact.
Do you feel that your ex doesn't deserve to spend time with your child?
Or rather that you child doesn't deserve the opportunity to develop a relationship with both parents equally?
What you're describing isn't ideal, but he won't get better at being a parent without practice. I'm sure you don't always get things right or know what to do. Maybe you even defer to asking a friend/relative for advice when you're not sure; someone you feel might have some wisdom to share...