It sounds similar to when our daughter had croup. Evening is usually worse.
I'd really caution you AGAINST cough medication until you've checked with your pediatrician or pharmacist. Last year, the American Academy of Pediatrics recommended that children under 6 NOT get cough and cold medications.
We did use Delsym, but only under the recommendation of our Pediatrician, and then, it was very sparingly.
For croup, the best thing is a humidifier and cold temperatures which help open the respiratory system.
I would call the pediatrician (even if it is on Christmas Eve - it should be a simple call that won't interrupt festivities at this point, and it will give you needed peace of mind).
Good luck! I hope he is well soon.