I think it's important you be honest with your partner. You can support him just by being by his side, but also focus on limiting the amount of stress you're experiencing. It'll be a balancing act, for sure, but I think you also need to realise it's normal to feel sad or overwhelmed when someone you care about, or someone who is related to the person you care about is going to die. It reminds you of a lot of things that invoke deep feelings and it's healthier to try and deal with it and go through it now then medicate it or numb it away. These feelings won't go away with the use of alcohol or drugs, it will just put it off.

Let yourself express your emotions. Don't bottle it up, try to numb it, etc. Feel free to cry, write things down, confide in close friends/your partner; don't be afraid to feel angry or upset with what's happening.

I think my signature really applies to situations like this:

"You gain strength, courage, and confidence by every experience by which you really stop to look fear in the face. You are able to say to yourself, 'I lived through this horror. I can take the next thing that comes along.' - Eleanor Roosevelt"

I hope for all the best for you. I'm sorry you're going through such a difficult time.