Are you able to talk to the owner of the horse about some of the signs of possible founder that you are seeing? Other than that, I think you’re doing a great job by giving him so much exercise; that probably has helped him so far! Is a grazing muzzle an option? You mentioned that he has 24/7 access to hay, which depending what type of hay that is, a grazing muzzle could help decrease his intake which might help decrease calories/sugars consumed. I hope you find a solution!