If they are turned out 24/7 you might start using the grazing muzzle during the day and taking it off at night. Traditionally horses consume less turned out at night is the reason it is recommended to use the muzzle during the day. If needed then you can switch to a lonfer muzzle wearing period.
Are you feeding anything else? I know it's probably old news but even though he is over weight he still needs the proper nutrients to avoid other issues. This is a good product if you are in need of something. IR Pellet