Reducing the amount of grain or switching to hay with a lower calorie count will help.
You can less let your horse in the field so he can less eat grass and go more often for a walk, do more work sessions to compensate.
Hi Dereck, the best is to start by reducing his ration but do it slowly. The horse has to still eat enough of course.
Then, adapt his diary planning of work. If you cannot work him more, let him in a paddock (with no or a few grass ideally) so he can unwind.