How is his intake?
If his intake is less / if his breathing is fast/ has difficulty in breathing/ becomes lethargic please take him to ER immediately.
If he has cough
alone with no other symptoms mentioned above, you may try giving him Plain Ambroxol
syrup (mucolyte) 1.25 ml three times a day till you see your doctor.
But since he is a small kid, its better that you have him examined by a Pediatrician
at the earliest as other medications can be decided only after examination of the kid.
Hope you find my answer helpful.