Spot has the most solid chest I've ever seen, but his rump leaves much to be desired... he's got a really flat croup, I don't know if that's atypical of paints/QHs ? He's also got a bit of a roach back. Could that be the result of him being so hollow? Or maybe the cause? Go ahead and have a go at his overall confirmation. He's got a lot of running blood on top in the paints / QHs, and on the bottom his momma was an appendix (he is 1/4 TB). I noticed he is a bit sickle-hocked, any other opinions would be appreciated.
THe one things that I notice that you haven't stated above is that his neck appears to be built a little too high up, giving it the look that it is too long.
I'm not really certain if there is any way to fix a roached back or even what causes it but his croup isn't as terribly flat as you think it is ;)