Have a look in his mouth- white "koplik's" spots can be seen on the cheek lining if it's measles- usually from day three (after the cough). (This is the best way to diagnose measles- but they only last for a few days- they look like grains of salt with an inflamed red patch around them.
If you're worried call the doctor- especially if he has a possible immune system problem. In the meantime try to get some vitamin A into him- I've given some references for this in the MMR thread. As much vitamin A as you can- although not megadoses as it can be toxic (not to worry you on this either- I just mean don't go crazy with it!).
The measles rash starts 3 to 5 days after the cold symptoms- usually as the fever goes up. the rash starts around the ears and face and then spreads.
Homeopathic remedies which might be useful include aconite- and belladona (especially useful if eyes are red). (These remedies would be useful for fever - even if it isn't measles).
Has DS been in contact with anyone with measles?
If you're worried about calling the dr- try NHS direct for advice.
Don't worry too much- even if it is measles- remember it's usually a mild disease (especially in this country where it's been around for generations). But if you're at all worried speak to the dr- at least they would be able to give a firmer diagnosis.
BTW my son had similar sounding symptoms before xmas- he had the fluey bug thing. His temperature was high for 5 days- but no problems.