I am the only one who thinks buying a child 50-100 toys a year too much?

If you find what your mother is doing, I would make arrangements to go out with her and your son together. this way you are there to moniter what he's getting and what she's feeding him. If she doesn't listen, politly explain to her your reasonings for not buying him so many toys and if she doesn't respect that, she can't see her grandson.