Hi @maryflorida, I think the course of friendship, intimacy as well, if you want it, is the best one to pursue, as everyone has pointed out. My husband was diagnosed with Alzheimer's in 2019, but I noticed something amiss two years earlier. Generally, he has a good disposition and is highly functioning. He has a low tolerance for visitors these days (about three hours max) and gets jealous when I'm talking with friends on the phone for a period of time. And since our life is much smaller now and I don't go out much so I can be with him, I look forward to the few conversations I can have with friends.
The thing about these disorders is we never know how they will go. Your friend could be loving and gentle now, but angry and abusive in the future. He could totally forget who you are. He could start doing strange things. If you marry, he will be your responsibility.
You are very kind and caring and he is fortunate to have you as a friend. You are probably his anchor now as he faces an uncertain future.