You need to add more quality calories. For your age, size and body type, you probably have a very high metabolism. Add at least 2 more full meals that includes protein and carb. don't worry about too much carbs, you look like you can handel all the carbs that you want to consume. Most important to gainsize is pack on the protein, lift heavy, cut back cardio to 2 times per week. Try this for about 6 weeks and see how much you pack on. Good luck, bro.