I don’t have any advice for you but I have the same thing going on with my rising 11 OTTB. Just started him on adequan which seems to be helping slightly. (only 3 doses in) He too seemed to sort of mope and was unwilling to move and the final straw was when he tried to buck me off kicking with one leg then the other. My vet said do the adequan, ride normally (for me that’s not very demanding) and see how he does in a month. Disappointed to hear your adequan treatment only lasted 6 weeks. Looking at supplements but hard to find any of non bovine or piggie origin. Still looking….
Good luck with your horse – it’s very sad when they don’t even feel like playing with the other horses