Approach a local benefactor such as a corporation or wealthy individual and see if they're interested in sponsoring the trip.
There was a young climber in our area years ago who had earned a bid to a world competition but couldn't afford to go. He was motivated enough to go that he approached local businesses and found sponsors who were willing to help fund his trip in return for the publicity the story generated. He is now one of the top climbers in the world, but it wasn't the trip that put him in that position - it was the drive to work for what he wanted despite the odds in front of him.
If you can instill a belief that it is within your son's power to set a goal and meet it no matter how long it takes or how hard it is, that lesson will likely be far more instrumental to his success than any one experience he may or may not have due to limited available funds.
