Kids of this age group can have 6 to 10 episodes of upper respiratory infection
per year- with cold, cough, fever
Kids sent to day care/ play schools etc are more prone for frequent infections.
However if cough is associated with wheeze, precipitated by a viral infection, you may approach your doctor for preventor medications.
His weight seems to be okay. Just give him a nutritious diet with plenty of fruits, vegetables, eggs daily.
You may also get micronutrient
supplementation after getting advice from your Pediatrician
Teach him hand washing techniques.
Hope my answer helps you.
Wish your son good health.