Sorry...not sure of a tasty cough medicine. We use a basic "Tussin" (equate brand...can buy it at Wal-Mart). Our doctor told us with NO letters (meaning NOT Tussin DM or anything like that), BUT your son is probably old enough that the 'letters' won't hurt him. I used basic Tussin while pregnant, because it was one of the few things that was safe to use.
For the cough, I would suggest that you try Vicks Vapor Rub on his FEET at bedtime. Lather it on the bottom of each foot; put socks on if you want to prevent a mess; and you'll be surprised how well it works to stop the coughing at night.