Is your son unhealthy? Or is he just small for his age?
My ds6 (almost 7) has always been a little guy. The “normal” weight on the weight charts at the pediatricians’ offices are an average of all different children together. Some children are naturally smaller, some naturally larger. If your ds has a smaller frame or high metabolism, for example, he’s probably always going to be small (which will be a blessing later in life). That’s how my ds is. He’s totally healthy, eats great, goes through periods where he’s not hungry, and then through spurts where I can’t buy enough food to keep him full.
He will eventually be the size God intends for him to be. No worries! 
Also, I hesitate to say that making meals full of fat and calories ahead of time is the best solution, and here’s why: You want your ds (and the rest of your family) to get good nutrition, not just meals full of fat and empty calories. Pasta with cheese sauce is definitely a meal full of fat and calories, but what is its nutritional value? You said, “Both hubby and I are starting to feel the effects of eating meals like this every night.” So, does that tell you that maybe it’s not the best thing to be eating? If you’re gaining weight, feeling tired and sluggish, lacking energy, etc., it’s probably having some similar effects on the kids.
I’m not trying to criticize what you’ve been cooking, so please don’t take it that way. I used to try to stuff my small child full of stuff, and he never gained weight, until he hit a growth spurt. We started adding good fat to our diets a long time ago, and we all feel so much better than we used to on our diet full of carbs, cheese, and empty calories.
Olive oil, real butter, avocados, almonds, and salmon are all foods full of good, rich fat … not the kind your body stores and then can’t get rid of. A high protein diet will also help your son bulk up without filling him full of nutrition-less calories. We eat lots of veggies roasted in olive oil in our house, and we don’t feel as bad as if we had just eaten four slices of cheese pizza. How we feel is sometimes more about what we’re eating than how much we’re eating. If we’re eating junk, that’s how we’re going to feel … which, I’m guessing, is how you and the hubs are feeling.
I realize pre-made casseroles are really convenient (I have a freezer full myself), but there’s no substitute for good, fresh foods, rich in protein and not just empty, fattening calories.
Blessings,
Lindsey 