Sounds like my grandmother, in terms of accepting my AS. My advice would be to simply avoid talking about it, and when it's unavoidable to take your time in making sure you use the right words to convey what you're trying to say.
Easier said than done, but some people just have difficulties accepting conditions, especially when it concerns someone they care about. They don't "see" it and so, in their eyes, it doesn't exist. All you can really do is be clear when you talk about it and give him time to process it. Whether he accepts it or not is anyone's guess, for your sake I hope he will.