I can’t say whether or not it would work, but it might be better to broach the subject of your concerns about your future and see where that leads, rather than this specific suggestion. If you were to live so close to your son, are you assuming that he would become your carer eventually, or his wife, if he has one? That is a big step, a huge responsibility.
Another thought is that a house with an annexe could be hard to sell, in the future.
Then there are the legal and financial complexities, on which both parties should seek advice independently. Such a decision will also affect your other son when it comes to inheritance, as some of your estate will be tied up, possibly for years, in this property.
Another aspect is the social side. Do you know anyone in this place? Will you feel isolated in your annexe, leaving all your friends behind?
My daughter has a house with an annexe which is currently a holiday let, so I have thought about this possibility too.