It got worse as from 20 months until about 23 months, and I was beginning to think I had the ugliest, gangliest looking little miniature horse on the planet. Ewe-ish neck, mutton withers, tiny muzzle and huge head, and about 2 inches butt high. It wasn't until the past four or five months (she turned three years old this April) that she hit her last (or atleast I'm hoping her last!) growth spurt; and her withers and butt have evened out, her ewe neck has long since dissapeared, the flat back is mostly gone, and I believe that her shoulder slope has even improved a bit. A lot of horses don't fill out until their third-fourth year...which is why it's best not to ride them until then. I've seen it happen quite a few times.
Fourteen months: very gangly and odd looking, but not butt high
Two years, a couple of months (not sure exactly how many) VERY butt high
No current pictures though :3 I'll have to get one.
OP, you've been given good critique. He has his issues, but really is a solid, well built little guy. I don't see him having too much of a problem with 2'6 hunters under the right rider ^^