My teen with weight problems?

I know this is an issue for many young kids, including my boy. He is 1 years old and about 5' 7- 5'9 and he weighs about 85 pounds. He is very self- conscious about his weight, and he wants to try to gain more weight. Can anyone suggest a dietary plan for him, of what he needs to do and what foods he needs to consume and how much. THanks