My son is 17 months oldand has chronic runny nose, watery eyes, coughing and sneezinq. He has ear tubes, and uses a albuterol nebulizer that is used as needed. He has had RSV twice once at 3 weeks old and again at one year. He has been labeled as having a reactive airway.
The best person to answer this question will be your son's pediatrician or ENT doctor or whoever is directing his health care. There are allergy medicines available that can be used with children as young as your son. But with his medical history, I wouldn't advise that you try to treat him yourself with over the counter medicine.
First of all, his allergies have not even been diagnosed. Second, you'll want to be sure that any medications that are used will not affect him in a negative way, given his history and other medications he may be on.
So, talk with your son's doctor for the best advice!