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They don't all lose a lot of weight as they age although most lose some, they don't tend to overeat anyway so it's never good to cut down on food they need to have a good supply & a hoard available all the time.
He probably isn't overweight, hams vary in size a lot so he's more likely just a larger ham, he will probably eat a bit less if he does become a lot less active but you can still leave plenty of food out without worrying about it.