Long trotting, ground poles and trail riding/hill work
Maybe decrease his ration but before look out if the food isn't too heavy for him! If it's too difficult to work out for him because of his breathing, try to walk as much as possible, it can only be positive :)
I've read on an article that horses who're gaining some weight are sometimes a bit stressed, because of the lifestyle, his work, his health or even the food ... I don't have every detail but it works in the brain when they don't have whenever they want some grass ... As they've limited quantity, they're producing all the time some fat, because they're afraid of missing food.
You can go out, doing trottings, if think it's the thing which works the best! It's also good for your horse's mental :)