Firstly how old is your son?It can be very dangerous to put a child on a "diet" for physical reasons as well as being very damaging to their self esteem.
Anyway diets make you fat as they cause hell with the metabolism.The only sensible and effective way to lose weight and keep it off is to alter your eating habits to ensure you are eating healthily with plenty of fruit, veg,carbohydrates(bread, pasta, rice, potatoes, pulses) lean meat etc.
Basically cut out all the fatty, sugary stuff.
Also getting more exercise will speed up the metabolism which will help to get you fitter.
One thing that really helped me (I was obese from five until my early 20s) was to learn to eat when I was HUNGRY and at no other time,i.e, not when I was bored, sad, lonely or anything else.