Well, you have to stay happy for him, even if when you're on your own you break down completely. I found that seeing me upset made my mum more upset. He probably wouldn't want you to put your life on hold for him, so long as you see him quite regularly. Keep the conversation as normal as possible, don't talk about it too much. Be cheerful and laugh together, seems silly and pointless when everything feels so bleak, but scientifically even just a forced smile boosts endorphin levels. Also, try and avoid reading the papers too much, too many horror stories that really didn't help.
I found myself thinking that there's no use being down and depressed all the time (though obviously it's easy to feel that way and I did cry a bit most days) because the reason we all want to live as long as possible is because we want to enjoy life, so if it is the worst case scenario, don't waste whatever's left being sad, make it count. Same goes for anybody really, as we could all get hit by a bus tomorrow at the end of the day.
Just be there for him, it's all you can do. Sometimes you might end up feeling like you're the parent, so find somebody you can talk to and cry on as well, at the time I leaned heavily on my boyfriend and luckily he was great, but a friend or your mum or whoever that you can turn to when it gets too scary is invaluable.
Rambling, disordered mess of a post this, sorry.
I'll cross everything for good news x