He has solid bone, a solid mind, and, although he has faults, with proper work there's no reason he won't be a great trail horse/ fun horse. Because of his build I would definitely work on strengthening his top line and back, though. Going up and down hills is one way to do this- it makes him engage his hind end and use his back. There are plenty of exercises you could do with him.
My mom's mare is built similarly (I've attached a picture of her so you can see how she's built). She's in her 20's, and doing perfectly fine. She has arthritis in her shoulder and hips from her old age, but she can still be ridden on the trails and loves it very much. So, for what you want to do, I think he's perfectly suitable.
An important thing to consider are his feet, though... Have you gotten to look at his feet yet?