Asalaamualaikum my beloved sister in Islam!
its sad to know that brothers sometimes can end up changing their mind. sister you're not yet married to the brother, and remember being married does not mean that we Muslims can overlook our duties towards our parents. now I can see that its really hard for a woman because she is stuck in between 2 commandments of Allah (swt) so the best thing is that she cooperates with them both and finds a middle way, and for you the middle way is that, that you try educating your finance about you're duty towards you're mother, see when Allah subhanah says that we must look after our parents when they reach their old age, he doesn't say it only applies to men, so its a general commandment for both men and women.
I know its going to be hard for you to read my advice and u might not even find it effective or helpful, but my advice is: "is the guy forth your struggle and time if he has no respect for you're feelings and duties towards your mother?" would it do justice if you had said the same about his mother? sister speak to him if he doesn't agree upon your duty towards your mother, ask him if he can leave his mother forever for you? if he still insists that he doesn't want your mother, then think again, your mother is the woman who brought u up with hardship, your mother is the woman who's mahram lives away from her for month leaving her with no support or help, your mother is the woman who is overlooking her future for the happiness of her daughter and advicing her daughter to concentrate on her relationship. sister I would say, Engagement is not marriage, engagement is a platform where you'd know if the potential partner is happy to take you with full package, if he's not then I don't think he's worth your second thought, I'm really sorry I know it sounds harsh and perhaps you're attatched to him as well, but he is not your husband yet you do not have to obey him sister, would you want to obey a man who doesn't want your mother? please talk to him and see what he says try convincing him for some time and if nothing works the best thing for you is not to marry a man who shows no interest in your family.
I'm really sorry if I've hurt you sister, but I've advised you from my sincere heart and sincere well wishing for my sister in Islam.
your sister in Islam!