Hi Kids of this age get frequent respiratory infections. Those sent to play groups and day cares are more prone as it spreads easily among kids through air and fomites- like toys etc. If every episode of upper respiratory infection ends up in the chest- its likely that he has hyper reactive airway disease. Any viral infection can precipitate wheeze in them. Any allergic reactions too can precipitate a wheeze. Wheeze in the chest and breathing difficulty are not always needed to diagnose hyper reactive airway disease. Cough associated with wheeze is usually more at night and early morning. But wheeze should respond well to broncho-dilators like salbutamol. If you are using MDI with a spacer, be sure the technique is right. You may use mask attached to the spacer. Nebuliser will be better to give Salbutamol to control such episodes than spacer. How ever its better to have a look at his throat and rule out Adenoids. If its just Allergic Rhinitis without wheeze- medications like Monteleukast will help in prevention of such episodes. Steroid nasal sprays will also help. Kids overcome this allergy as they grow. Keep him off allergens like pollen, dust, smoke, perfumes and other strong smells, temperature changes, animal danders and other food items he is allergic to. Teach him hand hygiene. Give him all the needed vaccinations after discussion with your Pediatrician. Hope my answer helps you. Wish your son good health.