Take him to the vet. They need to be able to see him physically to determine if he is overweight, by how much, and what needs to be done. It is hard to say without seeing him, because even dogs of a certain breed can be bigger than the standard and not be overweight. For example, my Min Pin has a much larger build than most Min Pins, so if she weighed what those teeny tiny Min Pins weigh, she'd be under weight. Also, each breed carries their weight differently and some have a tendency toward obesity. Five months is still young. Has he been to the vet? Is his belly distended? It could be for reasons other than being overweight, like worms or something?
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