You may think you eat a lot..... but you need to eat more in order to grow. Up your calories by 30% and see what happens.
Keep your diet simple oats, rice, pasta, tuna, chicken, beef in large quantities.
Try and have at least 6 meals a day it will be hard at first but your body will get used to it.
If you want to buy something to help you though buy some whey protein or a weight gain shake like Pro-mass by CnP.
And drink a lot of water! (more that you think)