How much should my boy who is 8 years old weigh?

A mother of an 8 year old boy asked me for advice for an 8 year old child who she believes he is gaining weight fast lately. His height is 126 centimeters and his weight is 36 kilograms. Her question was should I put him on an exercise plan?( I don't give him junk food or let him overeat and his current physical activity is playing with his friends.) Any suggestions