1 solution, just before you go to sleep eat something with high carbohydrate content, like: pasta, rice, bread, etc...
People will tell you to drink, this is a bad solution because it lowers testosterone levels, this will decrease growth dramatically.
If you want to gain mass by muscle, lift heavy weights, this will not increase your strength as much but you will become larger, faster. Also eat a lot of protein before and after work outs.