Sorry to put a damper on it but from how you describe it, you might be about to commit insurance fraud.... if you knew or suspected he wasn’t 100% sound before taking out the policy it is a pre-existing condition and won’t be covered. Be aware that the insurance company will look at any lameness claims taken out right at the start of a policy with a fine tooth comb and may refuse to pay out leaving you with the bill.
Sorry if I’ve got the wrong end of the stick, but just saying how it sounds.
No insurance company will pay for a pre existing condition. What you have described is exactly that and it would be fraud to try to claim for it.If your horse needs the vet then he needs the vet regardless of who is paying the bill.