The only breed-restricted showing classes are for M&Ms, and we can see he's definately not one of those!
Might make a nice hunter pony? Or depends what level you want to show - locally there are lots of generic classes than any size/shape of horse can do.
You could also do CHAPS/BSPA classes, as mentionned above.
http://www.chapsuk.com/
http://www.bspaonline.com/