"I've been diagnosed as having Alzheimer's"

S.... told me yesterday.
He had been sitting, as he often does, in a chair by his front door. He had given me a wave and  I had stopped briefly to speak to him. I know it had taken all he had to admit this to me. 
S...lives alone. He has never married. He worked for the further education sector but had been retired for about ten years. In that time he has done very little. He reads and he gardens. He goes for solitary walks. Occasionally, very occasionally, he goes out with a nephew who lives on the other side of the city. His sister lives in another state. 
I met him after his very elderly neighbour, now deceased, introduced me. S.... was going into hospital and she thought he needed to know that I was the person who sometimes took her prescriptions to the chemist. He didn't want me to do that. He was fiercely independent.
    "I don't want to be beholden to anyone," he had growled.
    "And if you don't let anyone check on you and you end up on the floor then you will be an even bigger nuisance," I told him. I sensed he was the sort of person who had to be told that. He was. He agreed reluctantly that, if he needed help, I would do the chemist and the milk run.
He asked for help twice. I left prescriptions at the chemist. I later discovered that he had bought a supply of "long-life" milk to see him through his convalescence. 
His neighbours have never been into the house. I have never been into his house or his garden. I simply picked the prescription up out of his letter box. He has never directly said please or thank you but  he did start to talk to me if he happened to be out working in his garden. Three years in a row though he gave me his lavender cuttings with a rough, "You might be able to use those."
And now?
I don't know what he will do. He isn't the sort of man you can ask. He knows I will help if he needs help of the sort I can give. It must have cost an enormous amount for this intensely private man to admit  his problem to me. I certainly couldn't tell him that I had wondered recently whether there wasn't a serious issue there. He had seemed confused and forgetful and he hasn't fully understand the diagnosis of another serious condition.
I wonder what will happen to him. Will he go on staying there to the point where he can't care for himself and the house, which is not in good repair anyway, starts to fall down around him? Will his nephew, a busy man by all accounts, have to try and help? It won't be wanted. 
I know I'll need to go on making contact when he wants it - and that won't be often. How long before he "forgets" who I am?
How do you help someone like him?