This is really tragic and I am very sorry you have to go through this.
I've never known someone that had a death sentence. If you believe in God I am sure praying would make you feel better.
Like everyone else has said I would enjoy every moment you have with him, not let the petty things get between the two of you, and tell him you love him (if you do) every chance you get. Don't treat him like a charity case.
If you need anything feel free to PM me, and I'd be more than happy to try to help you out.