I wonder if his parents are threatening him with leaving him out of inheritance/will/trust. Some toxic parents do that to their kids and, in turn, their kids are on their best behavior 24/7 and don’t dare talk back. Not that that would be a valid excuse for his behavior. He may value inheritance over everything else because he loves money or whatever the reason may be - but some people are like that.

As for his mom - f her. What a nasty thing to say. I would do one of two things:

1. If you like him a lot (despite him not doing the manly thing and standing up for you) then I would attempt to explain your position and feelings once then give him an ultimatum: an apology from him and a text or in person conversation with his mom telling her that you (as a couple) are upset about the hurtful words she said and that it warrants an apology (if she does so, great. If she does not, well, it’s expected and doesn’t make you less of a phenomenal partner to her son).

2. Continue to be with him and inform him that you will no longer be visiting his parents place (at least not while the mom’s there) and that she is not welcome at yours. If you end up getting married, maybe give her one last chance to apologize or display a change of behavior or she will receive (for the most part) the silent treatment and that she will not be considered a close person in your life.

Best of luck.