How to help a fat kid?

A dear friend of mine has a son who has put on a lot of weight the last 3 months
He is worried about his kids health and asked me for advice

I told him the typical stuff , watch out for sweets and unhealthy food and make sure your kid stays active. But because it has been excessive he is worried

Is there anything that contributes to weight gain in children besides poor diet and inactivity?

What are some things to look out for?
How to help a fat kid?
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