I would stress honesty and communication. Don't pressure him to open up to you, that's something HE needs to do in HIS time. Any pressure or guilt on your part will make this scary time for him and his family MUCH worse. Him not opening up to you does NOT mean that you don't have a good relationship, this is how HE deals with things like this. <---You really need to take this to heart.

I would just let him know that you are here for him, in any possible way. That you are here if he needs to talk, yell, cry, or just talk about a subject so far away from his brother possible, etc. Just knowing you are there and care unconditionally, is what he needs. Tell him that he doesn't need to worry about you at this time, he is free to spend his time, emotions, strength, energy on his brother and his family. That when things get better, you'll still be there too.

Put yourself in his shoes and I don't think you'll go wrong at all.

I'm sorry his brother has cancer. I hope and pray the surgery is very, very successful and that he feels lots better because of it. I'll keep you and his family in my prayers.

Hugs,
Kelly