I have been in a similar situation, except mine was with a 5yr old who was as close to me as my own children. He was dying from cancer, and lasted 1 week after the dr's told us there was no hope left. Of that week there was only 3 days in which he wasn't in a drug induced coma. But those three days opened my eyes to a world full of beauty and wonder. Every moment spent with him was precious and instead of thinking about the tomorrow and the pain of losing him, we all (his mom, dad, grandparents and I) just focused on the present. We built some absolutely beautiful memories of those last few days. And it is those memories I cling too now a year and a few months later. Caiden was able to give each and every one of us some special memory that I know if I had been thinking about the next few weeks/months/years without him I would have missed. My special memory is when he asked me to "sniff his butt" and gave me my own package of corn pops because he didn't want to share his. I have that package of corn pops in my jewelery box and everytime I see it, it makes me smile and remember him in a good way.
We didn't treat Caiden any different then we had, he was still a 5 year old who liked to pretend he was a knight killing dragons and some of his blows hurt enough for me to ask him to not hit.
Anyways what I'm trying to say is when you are with your friend, just be present, live in the "now", enjoy every single moment of being with him. There will be enough time after he is gone to mourn his passing.
I do truely hope that the drs are wrong and he has plenty more years, and I will be thinking of you.