How fast can a teenager gain muscle m?

Hi, I am 14 weigh 113 and need to get to 120 lbs. the doctor said for a healthy BMI . So I gain about a pound to 2 pounds a week...I walk like 3-4 miles twice a week and play basketball 2 or 3 times a week for an hour also. My weightlifting is 3 times a week for about an hour. Please help.....I also take Muscle Milk, Whey Protein, and Mutant M .