'Cancer' is a broad church. It could mean anything from 'he'll be dead in a year' to 'after a couple of ops and some chemo and he'll be back in the game'. There's nothing anyone can advise you on around that without knowing more (but don't post it. It's none of HN readers business.) Also, it's fair to assume you're giving him as much support as you can, so saying that isn't very useful either.

Hopefully you got some decent key person insurance set up at the start. If not, whoops.

The next thing to do is to talk it out. Does he want out? If so, can you buy him out? If not, is it realistic that he can stick around? Is there anyone in your network you can bring in to replace him (temporarily, if he's got a good outlook)?

If he can't work in the business any more I suspect you should approach this in a fairly similar way to how you would if he just wanted to leave for any other reason, or wanted to take a large block of time away from the business. Being dispassionate about it, you have to accept that the fact it's cancer is actually largely irrelevant; that's just the catalyst for action. As far as the business is concerned it's the action itself that's important.