Post legged means lack of angulation behind. In a severely post legged horse they can actually have an issue with their stifle dislocating (it can be put back by bringing the hind leg forward and out and pressing on the stifle joint to pop it back in).
The hind leg of a horse is the source of power. It is designed in a spiral like a spring. Correct angles enhance the power. Incorrect angles detract from the power. A post legged horse may not be able to collect as easily for dressage.. may not have as much power over fences and may develop spavins in the hind hocks. He will be short strided behind.
All that being said, there are many a post legged horse out there that has done just fine. This boy looks like he is full of personality and disposition and that is worth a whole lot.
My first horse was a Percheron cross and not as good as this boy.. he had an osselet on his front pastern that should have put him off riding. It did not. He won a 30 mile competitive trail ride (back in 1971) and did dressage.. even held a Levade for 10 seconds. He was not post legged.. his issues were front end.. and he as a Roman Nosed horse to boot. He was also 100% trustworthy and taught many a person to ride.. some of whom went on to ride at very high levels.
Attitude can make up for a lot. Just sayin'!
(do look up EMS.. I included a link).