A horse is worth what someone is willing to pay, and that's it. Which is why it's hard to value horse when you're from all over the world doing different things.
He seems like a sweet pony.
To me the main difficulty is finding what sort of market you'd be selling him to? He's too small for most adults and teens to consider, and sounds like he might be a strong jumper for a kid, and he doesn't seem to do dressage etc for what you'd want a kids all-rounder to be. If he's quiet and kid safe though, he'd always sell, people love horses that are good with kids.