Most show horses end up having their hocks injected as they go on in their careers. I've had it done on several horses, not all at once thank God, but it can really make a difference. It's expensive but it prolongs their rideability and comfort/soundness. I also like to have something like Legend done at the same time, I found that my horses did better, longer the hock injections were done along with IV Legend.
Only caution I have about joint injections is, make sure they also inject some kind of antibiotic at the same time, my vet uses Hyaluronic Acid, a corticosteroid, antibiotic and I think...maybe some kind of Novocaine for pain.
A person I know had their vet do hock injections and ended up having to do surgery because her vet did not use an antibiotic and the horse ended up with a joint infection. Her vet had told her that the antibiotic wasn't necessary. Errrr, I think anytime you're doing something THAT invasive, you're better to be covered.
Anyway, after he does the hocks, he injects the Legend, and we're good to go after a couple of days off. I've been lucky, following that protocol, we've only had to do the injections once a year. In the past, before Legend, I've had to do it up to 4X/year to keep a really good jumper sound.
So ask your vet how he feels about all that.