I would ask your wife to pick a couples counselor that you and she can go to to hammer out the details of the situation and when and how relationships with your mother can take place. It does sound to me like your wife is being a little over the top but could get better if you all went to an independent arbitrator.

You talk about how your wife feels about your mother and your mother about your son but not what your mother says/feels about your wife. If your mother puts her down, baldly or subtly, especially when your son is around with the two of you, that's not an acceptable. Your sons behavior should be set by you and your wife, not your mother, either. Spoiling by her, buying him things out of the ordinary, etc. is not acceptable either.

I would try to work out seeing your mother, yourself, mostly at lunch or other times when you are not "home" and when it isn't so obvious to your wife. I don't mean lie about seeing her, I just mean see her on your own time when it doesn't upset your previous life until you can work with your wife and someone independent on how to resolve this issue.