Is there a way to increase the size of the boot sole, or reduce the minimum size of the binding setting?
Hang on to the SL-11/Race-310 for another season or so until your young lad has boots to fill them. If he doesn't fit a 310, the SL-11 might not be a good ski for him yet.
With 3 kids racing, you should check out the local half-back programs. Skiis & Biikes, Sign of the Skier, and even Sporting Life have half-back programs for all kids, and they also have discounts for kids in registered race programs. That helps ease the financial pain a lot.