I do think you could spin it that your son has a "special condition," and that this means he has a controlled feeding with a very different schedule. Less that "doctors know better now," and more "Your grandson has a different special need.
You could also say, and I think it should be YOU, and not DH, "MIL, dear, it really bothers me that you won't drop this topic, and that you keep pushing it on us, even when we've told you that we are following *our* doctor's advice. I'll be honest--it makes us worry that we cannot trust you to follow our directions, and makes me very reluctant to leave you alone with the baby, or to allow you to hold him. I would hate for that to happen. So please, accept our decision, and stop bringing it up. So we can trust you. This is our baby, it's our turn to make the decisions."