You can't afford the trip and made the call not to go. There really isn't anything to talk to him about. I know you feel disappointed, but you had to make the decision based on lack of funds. You don't expect them to all cancel the trip because of that, do you?

Also, he should not feel obligated to pay your way. So there is nothing to say about that either.

Wish him off, tell him to have a great time, let him know you will miss him and wish you could be there, too. Spend the time doing some things you'd like to do just for you, visiting other friends, etc.