I'm sorry to hear about your friend. It's not at all selfish to feel sad about your friend's diagnosis, it is a sign that you care deeply about your friend.  Allow yourself the time and support you need so that you can provide the support he needs.  He's lucky to have you and I'm sure he appreciates that.

I recommend this book as a great place to start.

http://What Can I Do to Help?: 75 Practical Ideas for Family and Friends from Cancer's Frontline https://www.amazon.co.uk/dp/1906021562/ref=cm_sw_r_cp_api_i_BqCYCbM318RPR

 Tell your friend that you are there if and when he wants to talk, but know that he might want to focus his mind on other things instead, support him with this by planning enjoyable things to do, even if it's just watching a film together.

Sometimes practical help is the most useful, make specific suggestions rather than burdening with a non-specific offer of help.  Can you cook, mow his lawn, drive him to appointments or look after pets? Play to your strengths.  Let us know how you get on.