(I can't see the pictures right now.)
What breed is he? If he's a QH/Paint, it's likelier that he might stay butthigh. If not, I wouldn't worry too much about it. Horses don't really stop growing till 6-8, and many of the babies I've had have seemed to 'level out' at 3-4, and then hit a growth spurt again at 4-5 and then finally filled out and reached full maturity at 6-8. It varies per horse and breed however exactly how they mature however.
Speaking conformationally, butthigh doesn't necessarily mean downhill movement. It does put more weight on the horse's forehand and it makes it harder for him to actually lift the forehand to a degree that he can travel along nicely, but it doesn't mean that he's actually traveling in a truly downhill manner. Depending on the degree of butthigh, it might cancel out other good elements of his conformation, or it might only negate them to a slight degree. It certainly is an unwanted trait because of saddle fitting in self-carriage issues, but it's not a terrible fault like sickle hocks or post legged that will almost always cause some sort of soundness issues throughout the horse's life.
Being sore when going through a growth spurt is pretty normal in my experience. Not everything on the horse is growing at the same rate, so he's left in a slightly unabalanced state compared to a properly mature horse. Not to mention, the differentiation of growth rates in bones and tissues can shift the placement of certain things temporarily, until everything else catches back up. I'd have a vet check it, absolutely, but it's not a major turn off in my opinion.
"The art of riding is hard to find and easily lost."