You've got the right idea. Basically eat more. What you might want to do is add a carb & protein drink after any intense lacross games/practices. Do 2:1 high GI carbs to whey protein - Either buy some dextrose from the health food store and mix it with whey or combine gatorade and whey. That will help prevent any muscle loss and add some addition calories to your diet. If that isn't enough add a casein shake (like muscle milk) before bed.