First of all make sure it’s a significant amount of weight (with all the eating disorders out there) I would suggest limiting the nosh and increasing fun activities..
Be careful about how you address the weight issue if it’s something that he is made fun of. It will affect his self confidence. There’s a fine line between watching and giving the child a complex.
I would recommend speaking to your pediatrician, nutritionist and a personal trainer for advice.
It’s best to give everyone in the house the same healthy foods so the child wont feel different. Incorporate exercise and sport into the family’s activites.