Oh, DEAR, SB!
Unless your NG has a strong backbone to set appropriate boundaries with his mother, or the mother steps up and recognizes that her adult child would benefit from having a mate into his golden years, hopefully living way past her demise, it is not a great looking situation. At this point, any time taken from her is going to put you into a "taker" from her, not an addition to the family that would be highly beneficial to her son.
My MIL was moving in like this. She moved to our town when her mate of 7 yrs. passed. My late husband was her only son, and she definitely felt his role was to take care of her. My LH set boundaries with her, did what he needed, but on his time as he had a wife and young child to care for. After his death, she started saying things to our son to set him up as her care taker as he got old enough. I cut that apron string.
My parents were the opposite. Get training, education, and go and do. I did take care of my mother for years, as she moved to ME but she was independent and we used hired staff and then a facility for her needs. She had taken care of my father with alzheimers for years until she could not. She did not wish that for us, and especially, her young widowed daughter and grandson. I am thankful for all she did and allowed. It could have been very different.
My bestie talked to me yesterday. Her mother became ill, hospitalized in Jan. She lives 2 hours away and in a home that is not appropriate for her safety. And my best friend and her brother are carrying the load since JAN! Her mother refuses to move, and her brother flies in every other weekend because he can. It is crazy. My selfless friend is stressed, working, taking family leave, and her marriage and family life is now suffering. She is happy to care for her lovely mother, but her life is in a disarray as her mother won't budge. I feel for her but when it is okay for elderly parents to dictate what they want and have everyone else cater, set up for losing jobs, relationships because of their needs/wants?
I may get some negative feedback from this, but I have been there, done that and am thankful for my parents' willingness to not ask their adult children to sacrifice everything. I took good care of my mother up until the end, and her needs and mine were met albeit with help from others.