Both too loose poops, and too hard poops are not "normal". It may not just be food allergies (whether from IgE, IgG, or IgA reactions - mid-way down this post are ways to test for food sensitivities: http://itsnotmental.blogspot.com/2011/08/brain-health-cut... ), but it could also be a problem with Yeast overgrowth and/or needing probiotics.
Some medications, such as antibiotics can really do a number on our gut, and from there, cause inflammation, and even a progression of immune reactions due to the inflamed gut.
I had issues like your son and got more and more allergic and inflamed and had hives... started reacting to more and more foods... even ended up in the ER with an severe IgG reaction. Some doctors do not call that a "true allergy" yet the ER doctors had to treat it the same as an IgE anaphylactic allergic reaction. Anyway, a lot of my problems probably stemmed from systemic yeast.
Actually, it was my children who we figured out first - they had food issues. One now is close to having her Master's degree in dietetics. And I agree with GrammaRocks - learn how to make your family food according to your child's needs. I hated to cook, but it isn't such a big deal. It starts with lots of fresh, wholesome fruits, vegetables, and meats from quality sources.
I cannot eat gluten or dairy or eggs and was addicted to sugar and refined carbs... and I actually used to be a picky eater but now I eat MORE variety than I ever ate before! Even my husband who does not eat this way when he eats out (which is often), feels better than before.