Hi
Welcome to the MedHelp forum!
If your son is a small child then the pain in one nostril could be due to foreign body in the nose. This possibility should be eliminated first by careful questioning if the child talks. Consult an ENT immediately. Sinusitis, nasal polyp or a nasal bleed could be the other causes. It can also be a referred dental pain from tooth cavity in upper jaw. A dental checkup may be required. Other than that it can be a hair follicle infection, a boil or an inflamed turbinate of the nose. ENT consultation is a must. Take care!
Could be a zit growing on the inside of the nostril. I had them before. They hurt pretty good but will go away in a few days.