Hee hee, I hope you were joking on the twinkies and milkshakes.
I agree with everyone else that there is probably nothing to worry about and once you get the blood work it will most likely confirm that and help you to relax.
But in the meantime, may I suggest some fruit smoothies. They are yummy. One version is to juice a few oranges on a citrus juicer (very cheap at Wal-mart) and then put that in a strong blender with some frozen bananas and/or strawberries. My son loves this and it has lots of healthy calories. I'm sure you can find other recipes on the web. You can also add some protein in the form of whey if you want.
If he is eating milk and cereal for breakfast, then maybe you could add a couple of eggs (boiled or whatever). If he likes nut butters then there are a variety out there you could try - almond, tahini (technically a seed butter), etc. They can be put on toast, in a smoothie, on a celery stick, or diluted for a salad dressing.
Take care.