Ah, that changes everything. I would hope your trainer has a good track record of producing young horses that go on to be good citizens, and are ridden happily by amateur and junior riders. That will be a good person to start with!
Kindly, does it matter, at this point, if his lameness happened pre or post import? Are you wanting to pursue legal action, or would you sell him on and try again if it was “proven” that the lameness is pre-existing? Again, I’m not being snarky, these are real questions. To me, I would suggest dealing with the horse in front of you and not worrying about when the inciting incident happened. I expect you’re dealing with baby horse growthiness + not insignificant hoof issues (white line and thrush, no matter how mild, are no joke).
Core exercises (Jec Ballou is the gold standard, EquiBand system is nice too), lots and lots of walking on hills and over cavaletti, as much turnout as possible, extremely precise farrier work (not necessarily shoes, but a good trim is necessary), and TIME. Time to grow up and grow out of any minor issues - he may in fact grow out of the KS. Baby horses grow and have weirdness that pops up, and it tends to be shifting. You get the vet when it’s more than mild (bunny hopping, for example), or when it sticks around.
Perhaps. But some studies also show that early, correct work helps build strong bones and soft tissues to tolerate that kind of work later. This isn’t to say slap draw reins on and go jump 1.30m, but gentle hacking out and getting him working lightly on multiple types of surfaces isn’t a bad idea. Also, installing some amount of “work ethic” and manners on these guys can be helpful in the long run - late started horses (especially big ones) are a unique challenge vs ones that know they are going to have to cooperate with humans every day and expend some energy doing so .
TLDR: if your vet has cleared him to work under saddle, I’d recommend taking it slow and doing lots of turnout, working on manners and core strength. We used to subscribe to “one day a week of real work per year of horse age” aka 4yos work 4 days a week, but they all got handled and turned out daily. If your trainer is good with the youngsters and happy to help you take him the slow way around, that’s a great place to start! A conversation about goals with trainer (and vet if possible) might be helpful.