With local shows you could certainly do cob type, condition and possibly m&m depending on the description of the class he does look welshy to me is he not registered?
If you don't want to trim legs and pull and plait, I;d present him as a Unregistered Welsh Cob.
Par breds have to be pulled and plaites, you coudlalso fdo Riding Club Pony , WHP etc in M&M turnout.
That means plain tack, for M&M brown may look nice but is not essential.
He would be really tiny against full Small Hunter of 15.2 middleweight, and would need to be be on the Show Hunter regsiter for that.
If you did pul; and plait, he could do Show Hunter Pony , up to 15 hands, if you or a rider are Under 21.
I was going to say the feather and mane would have to go for the hunter class, I have a welsh D and struggle to put him in any other class other than welsh or m&m even though I think he would make a fab ladies hunter or m/w but then when you look at the classes he really would look out of place! It might be worth going to some local shows and seeing the standard for yourself and see where you think he'll fit.