My suggestion would be to offer a quick validation of his feelings along the lines of "it is disappointing when friends aren't available to play..." and then encourage a redirection to other options.
If you know that a situation like this is regularly a problem it may be a good idea to make a plan with him ahead of time along the lines of plan A, plan B, etc. I don't think it is a good idea to feed the mood by dwelling or hashing it through for hours, but I would not encourage you to skip over the mood or try to tease or joke him out of it. I personally wouldn't appreciate it if someone tried to tease me out of it when I was down because I couldn't see a friend.
I would encourage him to start to develop some skills of learning to change his mood. This book for kids might be of some help.
http://www.amazon.com/What-When-You-Grumble-Much/dp/1591474507 It is part of a really good series for kids. This book might be helpful for you too.
http://www.amazon.com/Optimistic-Ch...mp;s=books&qid=1262797605&sr=1-1
