A.,
i agree with Dr Heather. Your son absolutely has allergies. One person mentioned it was "Daycare nose". Even with constant colds, your son would experience some sort of relief occasinally from his symptoms. The fact that he has something going on non stop for a year is definately an indicator of allergies. My family had a similar experience in our old house before we moved in March. Since moving, we have all recovered (me and my children, my husband wasn't affected) and have identified the allergy as being one of a mold nature. It is completely possible that your son could be allergic to something in the daycare. Either physical or environmental. Perhapse the cleaning/disenfecting products. I highly suggest a skin test or at least two covering both the common and not so common possibilities.
I know how frustrating this can be. I hope you find the answers you're looking for. Also consider a homeopathic allergy remedy or two if possible. Children do so very well with those. I use them with my family with great success. And absolutely look into something to help boost immunity such as Vitamin C or even probiotics.
Best of luck, and Blessings for you and your son.
Rolinda