I note from your other thread you say he's probably around 8, and that combined with the fact that he's white, would lead me to say ridden bridle (with reins) rather than inhand bridle or halter. That is, assuming you can bit him in time. I'd go for a small hunter bridle, probably a 3/4" noseband, and 1/2" thickness elsewhere, you could get one from Fylde saddlery online, with a little bridoon for the bit. That will also do for the ridden classes later on.
I also think he's quite fine for a sec A judging from the photo I took from the other thread, and would be tempted to plait him up as a show hunter pony. I'd use a solocomb and scissors rather than pull though. That in itself should broaden what you can do with him, as frequently the unregistered classes are mixed with part bred M&M's. Either way his mane could do with a tidy up.
Class wise, for inhand you probably have a choice of best condition, M&M/Part Bred/Unregistered depending on the show, hunter pony if you plait, senior handler/junior handler, T&T and novelty classes. Ridden classes, lead rein, lead rein WH, first ridden, equitation, riding club pony, hunter pony, M&M/unreg/PB, T&T.
If you choose to keep his mane long rather than plait, you can put a single plait in behind the poll and leave it hanging down. It's Welsh thing to show off the head, I put ribbons in mine to match my tie.