Sorry but this may be long
my ex husband had a severe stroke and lost his speech, short-term memory, and ability to do the most simple maths or write more than basic sentence. It was 30years ago, but there was an experimental course in London for stroke patients in which they had 'lessons' from 9 to 5. It was incredibly intense , the idea being that the sooner the brain was stimulated, the better the outcome. It was also exhausing. However, I gather the results were very good indeed but with a high patient staff ratio, was deemed uneconomic.
So, I set up my own course at home. Made simple maths cards (yes, it started with 2 +3), and gave him some every day, increasing the complexity over time. Raided bookshops for suitable English Language books and the best I found were English for foreign students. Gave him short paragraphs to read from local papers, and got him to write simple descriptions e.g. how to make a cup of tea, how to boil an egg. At the beginning, he could hardly tackle any of these things but, over time, and with a lot of sweat and tears, he was able to return to work. Sadly it was only in a basic clerical job unlike his previous job as an International Finance Negotiator but....it was a job.The
There was no help from the NHS apart from half an hour speech therapy per week, but the waiting list was 6 months and thus far to late to be of any use.
Sadly, and I am sorry to issue this warning, the stroke had deeper effects that only became apparent over a couple of years. He went from being a funny, gentle, kind, intelligent man to bring cruel, abusive and devious.
I can only echo what others have said ie. seek help from wherever you can and, most importantly, as soon as possible. Get his brain and body working even if it seems hopeless, time-consuming, and difficult. The steps may be small at first but ,as you say , the brain can be quite adaptable and resilient.
Just read your additional info about the visual impairment. That makes things more difficult for both of you but be adaptable and think of how you can stimulate his brain verbally.
I wish you all the luck in the world.