Once the law changes to prevent no fault evictions that is easier said than done.
this house will become your dc home. He could, after living there for a few years, be reluctant to give it up to pay for grannies care home fees if faced with having to pay a market value rent or buy his own place
Hell would burn over before I bought a buy to let property for anyone with my retirement savings , especially not a family member. Rental properties (my ex had one so I speak form experience) come with a lot of constant anxieties and issues. Your ds is young, with adhd , has limited experience of living on his own and clearly struggled with money - sounds like a perfect tenant 🤦♀️🤷🏼♀️.
I have seen my otherwise fit and healthy father, suddenly need to go into care due to mental decline. He doesn’t have dementia, we don’t know what it is , but it is unlikely he’ll ever return home. Right now he’s getting nursing care in hospital but once they stabilise him it’ll be costing £1500 a week for him to home in a suitable residence to get the care he needs. He could live another 20 years. He simply doesn’t have mental resilience to deal with sorting out an eviction or rental property if he’d gone the route you’re suggesting. Right now he barely has mental competence for anything.
im also retired, I have a good bollus of savings. But my pension income is modest. I need my savings for future liabilities. I’ve earmarked £80k for care, but also earmarked savings for my dc wedding, grandkids (well I’m hopeful!), buying at least 2 new cars over remainder of my life, similarly refitting 1 kitchen and up to 2 bathrooms over course of my life, replacing roof tiles (on their last legs), complete retiring that needs to be done, a new boiler. In other words all the things that when you are working you have cash to save for, or to take out and repay loans on. I don’t have income for that. I will have to use my savings for that. This is why so many pensioners end up having to release equity in their homes in later life- they’d not accounted for these big liabilities that they couldn’t afford out of their pension income. Add to that, as you get older you may need more help around the home like a Gardener or cleaner. All this has to be payed for. Worrying about if I have enough savings to see me through till I die is a source of low level anxiety - even I’m in a situation that if I live a long life I’m likely to have to take equity form my home.
you do what you want with your money. Do not put pressure on your family to decide what to do with theirs. Your mother has acquired her wealth through her own circumstance which have nowt to do with you. Your mother is responsible for deciding how best to earmark her money for the purposes she wants or needs - if that means she seems to be sitting on it and counting beans, that is not unreasonable, selfish etc.
your son is just in his early 20s. Even when I was young I had a succession of twatty landlords who tried everything, including sexual harassment, from the age of 18 to 29 when I married and moved into my first home with my husband
My own ds are 28 and 30 and in rental accommodation with no sign of getting onto housing market- eldest is in London so about as expensive and stupid as you can get. If your son was in mid thirties, maybe you’d have a point. But he’s early 20s fgs . He has to learn to live on his own 2 feet otherwise he’ll be a n entitled what’s it all his life. Stop helicoptering him and back off- you’re doing him no favours in the long run.