Yeah, a 56 is fucking huge and the only place to find this would be a Tall & Fat store. Most normal retailers carry a couple sizes into the high 40's but never into the 50's. It's not because there isn't a market there for it, but the cost for such a thing to the retailer is higher than for a more "normal" waist size like in the mid 30's. There's just too much material.
As for him wanting a dress or skirt to be a pretty princess in, he's still going to need to shop at a Tall & Fat store for women as well because if I remember correctly from my clothing selling days the largest size that a normal retailer carries is usually a 16. He'd be looking for something about double that so maybe a 32?
And while there is a quick and dirty way of converting between the two, you'd figure that such a "smart" individual would be able to figure such things out on his own. Oh right, he's not smart so much as smarter than everybody he knows meaning he's probably surrounded by fucking cretins and just looking at his neighborhood there doesn't appear to be any reason to doubt that.