Good on you for standing up for yourself and for your son.
Now you will really need to be strong if the backlash does happen. You know that you are in the right and that no one has the right to speak to verbally abuse you, especially so called family.
If family members will not stick by you then you do not need them in your life.
As for your mother let things calm down and maybe then send her a letter telling her how you feel and what your rules will be for her spending time with your son.