What does it mean if my son always sneezes when he eats?

Difficult to say: It's difficult to say, but it doesn't sound like this is anything serious. People often sneeze due to allergies or inhaling some type of other allergen (smoke, scent). If it's with certain foods, it's possible this could be an allergy, but if that's the only symptom that's not likely. As long as there are no other symptoms (cough, wheezing, swelling) it's likely nothing to worry about.