I've never seen one used at the Disney parks (or one in real life, for that matter). You may want to check with Disney to see if they'd allow it as they have a policy against having children in things that get pulled, like wagons, and this may fall under that policy.
They may allow a buggy board (
http://www.amazon.com/Lascal-3G-Buggy-Board%2d-Black/dp/B000EN1K0C ) that he can stand on behind the wheelchair or something similar but it's worth checking first to avoid any disappointment.