Someone as thin and active as him definitely can use more carbs than the rest of us. Keep plenty of potatoes (white and sweet), bananas, and white rice on hand. And he needs waaaaaaay more protein. I'm shocked that he can keep his energy up between breakfast and lunch on muesli and yogurt. I know I couldn't. Try eggs, bacon, veggies, or even leftovers from dinner. Put butter and cheese (if you guys are ok with dairy) on everything.
Smoothies are a good way to get some extra calories for people who really need them, and a homemade smoothie with coconut milk and bright-colored fruit is much more nutrient dense than a plate full of pasta.