You can give him a spoonful of honey, dark karo syrup, or pulpy orange juice. It will help with the dry tickle in his throat. Sometimes I will let my kids have a dumdum sucker to suck on. (if it is in the middle of the night, I stay with them until they finish so they don't fall asleep with it). I also keep room-temp tap water in a cup next to them to drink if they get that itchy cough.
My pediatrician has ok'd giving triaminic nighttime cough or children's dimetapp (1 teaspoonful on the measuring cup that comes with it) if his/her cough gets bad. I have given it to him as needed since my kids were 18 months old and it is a real lifesaver. Please ask your doctor about that first though, my kids were both on the heavier side of their precentile ranges for height and weight, so I don't know if that makes the difference. Sitting in a steamy bathroom for 20 minutes can help, vicks vaporub on the feet with socks and a cool mist humidifier are all helpful too.
Hope this helps!
A.