As far as Beatles songs, early Beatles music is more upbeat. What about sending the song, "Help" along with a message something like, "I'm here to HELP you through your challenges. Can't wait for our next Paul McCartney concert!"

I find "Live Like You Were Dying" by Tim McGraw to be inspiring and upbeat and I have cancer. Making the most of whatever time you have is a good message for anyone. That being said, I'm not sure how it would go over if someone sends this song to you. Would it seem insensitive? Not sure. If he hasn't been told it's terminal, you wouldn't want to imply that it is. You know your friend, so... Maybe you could add a note that you're ready to share more adventures and good times together. ???

When I'm feeling down, sometimes I put on Herb Albert and the Tijuana Brass from the 60s. That music is so upbeat to me you can't feel down when listening to it.

Prayers for your friend.