The 21 month old Pinto is weak behind and has a dip behind his withers that will likely only get worse as he matures. This will make saddle fit a challenge. He has a low neck set in spite of him having his head up. He is almost ewe necked.
His legs are solid and that is what you want at this age.
As the old horsemen have said, "tops may come but borroms never." This means he may mature into a better horse through his body but the legs you have are the legs you have. Let him grow up a bit more and see if he improves. I have my doubts.. but would not be eager to toss him out without giving him another year of growth.