Trail riding okay, but I don't know about an english prospect with that upright shoulder and front pasterns. A higher set on neck can be good in dressage, but his is almost vertical. It might not be that bad, looking at the other pictures, but he's not set up well in any of the pictures to really see him well.
Are they just starting him slowly, or is there another reason he hasn't been started at 5? Just curious.
I have to admit, I looked at the ad because I was VERY curious to see what a Perch/STB would look like!