V.W.
Slather Vick's vaporub on the soles of his feet and then put socks on over them. Put him to bed.
It is an "old wive's tale", yes. But it has worked many times for my kids. The theory is that the menthol in the vick's triggers the nerves in the feet to send signals to the brain (hey! there's menthol down here!--lol, who knows), which distracts the brain (overrides the signals) from the signals it is receiving saying to cough.