OP, the short answer is grow a pair (you are his mum), the long is
You need to take her by the hand and ask her to sit down, explain that she needs to listen to you,
Dearest MiL, this is non negotiable, either you completely without complaint or opinion , comply with all of my wishes as regards to DS, or I will choose childcare for him that does, do you think you are able to do that?
If she says yes then tries to add a but, say no the buts are the bit that doesn't work for me or DS.
Walk away it will never work for you or your son who is the person you are meant to be doing your very best for.
if she stand over you when you are feeding, say "go away'
She has had children, but we all know a lot more since then, so unless she has up to date training, she is out of date, and there is nothing more dangerous than someone who is too arrogant to take on board new information.
Ask her if her MiL ran the show for her, and how that made her feel?
Explain you want to work with her, but unless she is able to follow your wishes honestly you can't.
Then it's up to her, you have offered, she probably wondering why you are such a push over.
If she has any questions send her on here, lots of knowledgeable people,
ask her to start a thread ,
1, will mitts deform my grandson's hands?
2, should I feed a rusk to my grand son even if his mum (DiL) says no?
3, should I stop the medicine the doctor prescribed for GS, even if his parents want him to have it?
tell her to ask here,
You need to be the one who decides how your son is raised,