He has some nice sized withers...I agree to make sure your tack fits right...Hvaing high withers can cause a horse a lot of pain with ill fitting tack.
He also looks toed out in the back but only slightly...I wouldnt say its a big issue...and he is a tad strait in the shoulder...
I personally love his head an neck though...but that may just be personal opinion lol
All and All I think he is a pretty decent looking gelding...nothing to major wrong with him. I like him... :)
As for building up that QH booty....lots of hill work will help him build muscle back there lol that's the best advice I have on that lol
As for him being over weight? I don't think he is....he looks good to me.