What a cute picture of your son :) I worked with families for years as a Parent Educator and licensed Pre-school teacher and I sometimes drop by this forum, since so many of the concerns everyone is going through were constantly brought up in the many parenting classes I taught for years.
Now, I am not a doctor, and I would certainly encourage you to call the Pediatrician's office to speak with the Nurses (in many places there is a triage nurse available to answer questions...decide if the child needs to be seen, etc...as I am sure you know...but, honestly when my daughter was little, I did not know that this service was available and so the first time I called the doctor's office, they put me through to the nurse right away). Anyway, I recall working with a few families whereby the first sign of allergies that appeared came in the form of skin problems. I myself have allergies, so I talked with my allergy doctor about the problem some of the Mom's in my class were noticing in their baby. In a few cases, it was a question of switching formulas unless the Mom was breastfeeding.
In an older adult if a cold lasts longer than about ten days and there is no fever, then it might be considered an allergy, but even then it is a good idea to call the doctor.
Since my husband does not have allergies and I do, I recall hearing that there was a 40% chance that our daughter would get allergies. Luckily she never had a problem. She did have a "cold" or something that lasted forever her senior year in college and she did get tested for allergies...nothing showed up as an allergent.
Well, to be sure, call the doctor's office on this one. It is so hard when those little ones have a nasty cold...and, you could certainly run your question by them to see what they think. It has been awhile since I have investigated this issue, but I wanted to share the little information I did have.
Good luck to you and your family.
We realized that my ds's cold was not a cold when the decongestant my ped prescribed just wasn't working very well. Also (like Mary said), he was prone to little bouts of eczema on his face. Since I have allergies (my dh claims that I'm allergic to air), the doc concluded that it's allergies. She told us to get some Zyrtec for kids and it's helped a lot!
My DD is 1 and she was having the same thing, along with myself. Runny nose, cough, watery eyes, itchy ears. The works, we even had fevers. Went to her ped, she said it was allerigies put her on childern's Zertec and Childern's Bendryal. It's working OK, but not clearing everything up 100%. Still a pretty runny nose, but congestion and coughing seem to be reduced. The doctor said it could be until summer that she'll need to be on these. She said the fevers were because it had turned into a sinus infection.