How To Gain 50lbs in 5 years (To Avoid being Underweight)?

My 11 year old Son Weighs 90 pounds (40 kilograms) and he would like to weigh 140lbs at the age of 17. He's 4'11" And He'll be At Least 5"7" At The age Of 17. And Also I Would Like To Loose 50lbs So, How DO We Do It? He would Like To Gain 50 (In 5 years) While I Would Like To Loose 50 (in 14 months)