Age is a huge factor here and for when you give a smart ass answer to the age question that pretty much makes certain you're not going to get any help. If you're talking aboutn a 14 year old kid and he's 5'3 and 124lbs I'd say he just needs to add 250 cals to what he's eating right now. He'll grow when puberty tells him to grow. If he's 25, the answer would be completely different. Just come with your questions with more respect and remember, you're the one looking for an answer, you're not owed an answer.
As a side note, a guy that weighs 123lbs running gear is an utter joke. He just needs to eat and grow to a respectable size where he at least looks like he works out before hitting anything stronget than creatine.