He's 18, 6'2" and 158.
Dude, go to Mcdonald's and order triple cheeseburgers till you can't stand it anymore.
Chicken sammies at any fast food. I like the Carls JR Chicken Santa Fe. I need to watch what I eat at my age, but a few months of eating anything you can WILL help at 18. See what happens. I'd bet if you lifted hard, you could do 20-25 lbs in 4-5 months.
I am not Pro fast food, but I will say at his age and as a beginner, he'll need some cholesterol and fat to start on the bulking train. Eat Clean for the most part, but it should not go all to the waist.