The first thing that comes to mind is a strength issue from not being ridden regularly in the past month. The second thing that comes to mind is a possible lameness, but don’t get too concerned just yet. Have you palpated his back and his stifles? Is he sore there?
I typically see cross cantering issues with draft cross youngsters, because those breeds are not built to canter, so they often have to be taught correctly. They often cross canter, or flip their leads before you ask for a down transition…and it’s totally normal. Because your horse is an OTTB, I would be more concerned about pain/weakness somewhere. Does he have back shoes?
Sore stifles are commonly associated with cross cantering. There are a few things you can do to help alleviate stifle soreness using wedge pads on the hind shoes. It might be as simple as that. If you decide to do a PPE exam, the vet will be able to tell you whether or not it’s a physical soundness issue. If he checks out ok, then talk to your farrier about putting hind shoes on him to see if that helps. Also ask about wedge pads.
I wouldn’t be too concerned. He’s 5 yrs old, he raced, and he sounds like a big boy if he’s a “tank.” That’s a lot of weight to load on that hind end when you ask for a down transition. It’s a typical evasion for him to swap leads behind, but it may be something you can fix.
Don’t give up on him yet!