Very kind of him to reply to you. Perhaps it is just my culture, but asking someone you don't know personally who has a potentially life-threatening illness what their prognosis is can be perceived as very rude. Stallman is sharing the information he wants to share and rest is none of our business.
I think it would be rude under different circumstances but this is information that he already shared semi-publicly and would've shared with OP had OP been able to make it. Also I get the feeling that Stallman is enough of a pragmatist to not get too offended about that sort of thing though I could be wrong.
I personally think the more appropriate request would be to first research instead of asking him to answer your question, and if incomplete, secondly to kindly show interest in him, at some further point, discussing this more in a public way when he feels ready to do so.
Obv he doesn't seem to mind the question. Just my approach.
