Hi, I know this is quite late to be posting in this forum and I hope all is now well with your horse!
If you're still having the same issues I would definitely say change farrier. I have had many issues with bad farriers over the years and only have a select few I now trust - bad ones make issues in the hind far, far worse. In the photos of his feet his heals are too low, bringing them up and keeping the toe shorter will solve the dragging and will help him carry himself correctly.
Your vet suggesting that he was never going to be fatter because "he is just like that" bothers me, I think if you haven't already get a second opinion from a vet other people you know trust.
He looks like a nice horse but needs more weight and regular massage and chiropractic work. He steps short in the right hind (doesn't follow through) this definitely does mean there could be an issue but it could be as simple as a bad farrier or a sore back.
The dip looks like it could possibly be a widows peak (rough coupling) or a roach back (both I am researching now, hence how I came across your post). I would say it is more likely a widows peak and nothing to be concerned about, but build his muscle but working him in long side reins on the lunge and feed protein rich feeds.
Hope all is now well!