Get a Vet Physio onto this, I've worked with many like this, you're looking to get the timing of the trot even on both legs, physio and correct foot balance can really sort this. Sooner the better.
There is a reason that the boxy foot is boxy, it is the passive weight bearing limb, but why?
Anything in that limb from the foot up to the shoulder could be the reason, I'd start with a thorough investigation and veterinary physio treatment. Then I would get an expert trim to lower the...
I did it with one of mine 2 years ago, such technical, scientific report I was left reeling. Honestly, it may be of use, but be prepared to need a science degree to interpret it and not feel completely overwhelmed by it.....as I was.
Usually you would see some relief from those injections. But in each an every case like you describe, the first thing you need to do is understand what a healthy foot should look like, then compare your horse's feet to that and make changes to achieve a healthy shape. This is the best way to...
The thing is, what caused the sidebone, as it in itself is a reaction to imbalance. That imbalance might be the true cause of the lameness.
Can you share the xrays?
Well it's going to need constant management at best, depending on what life you expect from such a horse, its likely to be a deal breaker.
I took one on for free, kept it barefoot, fantastic outcome.....but I'm a specialist.
I wouldn't pay more than £500 for a horse with this diagnosis.