Exercise, does he like any exercise at all, I would hone in on that one. You could encourage him by doing activities with him. I would also say to not draw attention to his weight gain at all, as he will already feel bad about that. There is nothing like doing things together. Good luck.
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Pegasus
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“Everybody is a genius. But if you judge a fish by it's ability to climb a tree, it will live it's whole life believing that it is stupid.” - Albert Einstein