A relative of mine has cancer, and I hate the fact that there's nothing I can do to stop it or control it. I feel so helpless because I can't do anything to fix the situation or stop it. He's getting treatment, but it's an aggressive kind. I feel like there's this void, and I'm sitting on the edge of it, in my mind.. and every now and then I fall into it. It's the same feeling I'd get if I were punched in the gut.

There's actually something you can do.
Your job now, in my opinion, is to cheer him up. I know, you might think ... : "it wont cure him. it wont stop the cancer. But it will help him eitherway.
A person with cancer has a huge trouble dealing with it and prefers remaining as if he doesnt really have a problem. So he'll prefer if you treat him as if nothing happened. Tell him jokes. How your day was. Any detail will do, if it's interesting enough. If you know him well you'll know how to do it.

That's all I can say

What a patient needs most is care and concern. What they want is just company that will encourage and spur them on in their battle against the illness

I'm sure your presence is what he appreciates the most.

Just testing out the site.... Goodluck to all