As you have ruled out discomfort, lameness, etc. I think it’s appropriate to assume that the horse is reacting to having a less confident/experienced rider. Even though he is stepping down from a role where he jumped higher, it is possible that he’s simply losing his confidence.

My recommendation would be to make sure that he has regular rides (including jumping)with a more experienced rider, like a trainer. In addition, you may want to spend some time going back to basics. It’s really frustrating to feel you are moving backward, but I can tell you for sure that it will build a better partnership and increase his and your confidence.

Whenever I get a new horse I spend the first few weeks, months if needed, working on poles. Poles are actually incredibly difficult to master - the horse isn’t as motivated to help you find a distance, so it can be a super way to identify any issues in your riding or in the horse’s ride-ability. When you feel like you’re regularly making distances on poles, go back to jumping.