Ah, yes I see what you mean. That's good.
We've had coughs like the one your ds has, with no eating and lots of coughing. We just got by, by giving him lots of anything at all that he would drink, calpol for the pain in the throat, and little bits of anything that he would eat.
In really tricky situations I default to dilute jamaican ginger beer and little bits of ginger biscuits, but small cartons of smoothie or fruit juice go down very well in general coughs and colds. Unless he has a temperature that won't come down below 38 the gp can't really do anything, and kids under 6 aren't allowed cough medicine.
Did you try the vick-on-the-feet idea? I've no idea whether that works, but the others here seem to swear by it.
There's always the pharmacist if you would like to look for other ideas, and if you're really worried then a trip to the out of hours gp might be the thing.
Sorry you're dealing with this. I know what it's like looking after an ill little person. Hang in there. 