As others have said, ration his hay. Do not starve the horse, as it can lead to metabolic issues. 2% - 2.5% of the bodyweight in hay per day is a rough place to start. The weight tape is your friend, measure him every week or two. Make sure he has a vit/mineral supplement and unlimited clean water. If you can, more portions distributed over the day are better, or a slow feeder net so he doesn't hog out and then has nothing to do for the rest of the day.
Some horses, especially those who are used to being pastured 24/7 can have a hard time adapting to the new routine, so an hr of grazing in the morning and one at night could give them something to do. Just reduce the hay accordingly.
As for exercise - "work the snot out of him" might not be the best strategy. Not only are you going to make him sore at first, but it might also make him sour, and we all prefer a happy horse who enjoys working. I'd build him up slowly, trot, uphills, cavaletti, that kind of stuff. I'd work him to a light sweat and then stop and do something fun to cool and dry off (e.g. Go for a hand walk). That might be 10min of trot initially, or it might be half an hr, let your horse tell you that. Don't work him till he's dripping wet or huffing and puffing. The intervals till he starts sweating should become longer and longer. Regular shorter work helps more than two giant sessions a week.
Also, despite the fact that he's never been sick, I'd get a PPE done. There might already be an underlying issue like low-grade chronic laminitis or something metabolic (Cushings? Insulin resistance?)
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