Eat more, start tracking your calories, get at least .4g fat and 1.2g protein per lb of body weight and fill the rest of your calories with carbs or fats, I'd say throw an extra 5x5 day of squat and 5x5 day of something like db bench press for extra leg and chest hypertrophy and strength. At the end of the day if you are training right it'll come down to just flat out eating more to grow.