Your novel was a good one - full of all the right information
I agree to not give up on him, especially if you're not enthused about any sort of jumping.
Believe me, please believe me, proper diet really does go a long way in holding back inflammatory issues. Which, in turn, will help hold the horse's weight down<---in turn putting less pressure on any spots that already have issues.
ETA: I am a little concerned that they mightve taken his shoes off so that they could attribute any lameness to that :/
I never thought of that but it's a possibility
Flax has done a terrific job of toughening up my horses' hooves. That doesn't mean they became "rock crushers". It means that my flat soled horses that has always been sensitive and now has metabolic issues, has stronger hooves than he ever had, once I put him on Omega-3 Horseshine and he is 25 years old - lol
I honestly would stay away from grains and especially run-don't-walk away from anything full of molasses, like sweet feed.
If there is navicular, that is no longer the death sentence it used to be. There are now many ways to treat it without surgery. I am not Navicular Informed, hopefully others can offer some input.
Your daughter being only nine means there's not much weight going on his back so that also helps. Hopefully you can get him conditioned enough and his hooves/legs in condition to where you can also enjoy a spin or two around the woods
At age 19, I still vote for a chiro. I could write a book on how the chiro keeping the sacrum adjusted on one of my horses helped speed the healing of his torn ligaments on the front. Everything's connected and while the chiro can't fix navicular (if that's what's wrong), having the entire skeletal system in proper alignment will also help reduce discomfort