On Hulk, the ride op has a stick, and that stick is to measure kids. I've seen a ride op get someone out of the train, measure them, and send them to the baby swap side. On the exit side of hulk, most people cut thru where the tv monitors are where you look at your ride picture. That's a way to bypass the height inspection, but they can still get you at the train. Same with Deuling Dragons, they would still check at the train because you can go down the exit side bypassing the guard, then get back into the regular queue by going up some steps.
The best shoes to buy are Sketchers at a sporting goods store like the Sports Authroity. They're the popular brand these days, and they have a large back heel. Buy the shoes 1 size bigger so they are not tight, then have the kid wear thick socks. Yes, these shoes cost alot of money, but the kid will like them, that's why you buy 1 size big.
I'm sure that in 6 weeks, he'll grow.
We have the same situation, taking a friend who is right at 54". We'll tell her mom to buy the Sketchers.