my son is 4yrs old and he is so slim and short and not gaining weight what diet should I follow???

A. hi dear Kids at this age are still very physical, but they learn in a more focused and less hectic way than when they were younger. These kids typically gain about 4–5 pounds (2 kilograms) and grow about 2–3 inches (5 to 8 centimeters) per year. An average 4-year-old weighs about 40 pounds and is about 40 inches tall.