((((Hugs))))) I'm sorry this is happening.
If his mom continues harassing you, give her the info for the closest AlAnon group meeting & tell her that there is no way she'll ever "hear" this from you, but that if she really seriously wants to understand things, she's welcome to talk to this group as they have experience & knowledge about this exact topic & might be able to help her understand it all better. I also would have no issues letting her know that I won't be putting up with her harassing me about MY decisions pertaining to MY life & that I'll block her number if she continues in this way.