My first question is has the vet been out to see him? Was blood work done? Thorough exam?
Next: what is his activity level, i.e. what are you using him for? Is he an eventer? Jumper? How many times a week are you riding him and what are you doing during your ride?
If he’s not getting any exercise at all or moderate use and the vet has been out and hos NOT made a recommendation of their own, I would ask the vet for ideas.
Enjoy your ride,