mister-a, I'm so sorry you're losing someone so special from your life. Making a "best friend" is quite an undertaking, & to lose one heartbreaking. No one will ever replace him. I've lost a best friend, and although I was blessed with another best friend later in life it wasn't the same.
The only advice I can offer is to accept that this is will happen--nothing will change that. Be as gentle and giving with your friend as you can be in whatever time you have left with him. Make every moment you have together a good one to remember.
Try to stay in the present. Don't lose any time with him because you're thinking ahead to when he's already left. Once he's gone, try to again stay in the present. It will take some time for both of you to adjust to the new, different sort of relationship. Wait and see what develops.
Keep posting here. PsychCentral is a great place to connect and hang out. Get to know a few folks and see whether any of the forums feel like a fit for you. This one may work!
Take care & believe me--you'll find many others on this site who will understand how upsetting this experience is for you.
Roadie