thinking of buying this horse and wanted some opinions on him
You see that with a lot of QHs as well. Most of it is usually do to youngsters having worked to hard to young and having never build the correct musculature for the job they were asked to do. In some horses if its happend for really extended aount of time(months to years), it becomes a muscle group that is extremely difficult to rebuild. I know of a horse in particular that has it quite severely and tho it doesn't affect his soundness, it's really unattractive.The dip shouldn't be THAT prominent, and tends to go hand in hand with a high wither and a slightly longer back, so when he's on flat ground, you're going to have to assess his topline for whatever discipline you're looking to perform. A supplement of MSM/Glucosamine and a good, licensed farrier with proven corrective capabilities can help fix the spacing in the hip, and keep the joint wet.
Next, I noticed he's not too muscled up. His behind and his neck look a little on the skinny side, and you can't see his "rib meat."
Otherwise, he's a pretty little guy, and has a beautiful face, and if he cues from leg, even better!