Why don't you consider selling him to the riding place and buying another horse. It sounds like he is happy and doing a world of good there? You're fine either way as far as I'm concerned but that might be a good option. Horses are cheap all over so you could find one pretty easily.
I've considered it, and horse prices aren't the issue (though he's bred through the nose and a fairly expensive animal, quality wise).
As iridehorses said, there are just some horses who can not be replaced. I've had him since he was 2 1/2, I trust him completely and totally. In college I once cried myself to sleep in his stall, only to wake up to my trainer informing me my giant horse was squished up on the other side of the stall, because he wouldn't lay down where he might hit me.
My son is too young to ride yet, but I know without question that when he is old enough to hold the reins I can trust my guy to take care of him and show him the world and freedom I first experienced on the back of a horse. He can trail ride all day, or play games with other riders and know, as I did, without question his horse is 100% trustworthy and will take care of him - even as he crosses the railrode tressle on a dare, tries to canter backwards, rides him into the ocean to swim, has his bridle yanked off by a friend on a trailride, tries to mount and dismount off the wrong sides/wrong ends (in other words all the exceptionally stupid things I did as a young rider).
My fiance has had both his legs broken at the knees - Ro is the only horse he will get on, because he trusts him. If I want him to ever ride with me, I'll have to keep him.
I know he's doing good there, but I also know he has a lot left to give my family as well. It may be selfish, but I want to keep him.