At birth, my mother could not feed me, so she gave me cow's Milk. I reacted badly to it, rash, mucus but more importantly hear infections.
Usually heavy mucus is Milk and gluten reactions. I found pollen allergies to me were more very watery mucus.
I suggest you feed your son with just about only Carrot (if possible organic) juice, puree etc, and any orther vegetables to complement the diet. It worked wonders for me at that age, and no wheat or any gluten of any sort. You can add some good fat (coconut, olive butter oils)
Also check he does not have any hear infections. If not you might want to do a hear candling, it is the only safe way to clear the hear drum and more importantly the small internal gap between the 2 hears, which when blocks really make all these allergic reactions much worse.
Play confortable classical music to quiten the stress, of mother and child.
Good luck, Alain