However, locally you might get away with doing both so long as you 'blinged' him up for the RH and went very plain for the Small Hunter.
I had one that went the other way........really a small hunter but if I trimmed him carefully and dressed him appropriately, he won many small riding horse classes too.
I would say small hunter. They really expect the small hunters to be able to moe these days, so just because he moves well is not enough reason to put him in a small ridn ghorse class. However, to be quite honest there is a lot of cross over in the two classes. I would really need to see a sideways pic slightly larger than the ones you have on here to make a proper decision. How much bone does he have and have you got a height certificate?