Any discharge around the eyes? (usually looks like they have been crying, the fur underneath the eyes a bit wet?)
I would go to the vets as soon as you can, if you can hear that his breathing is noisy he might have liquid or mucus in his lungs which is bad.. Vedra from the CCT suggests giving the piggy a diuretic, to shift the fluid, and then a weeks course (minimum is a week, no less!) of antibiotics, usually vets give Baytril.
Also, when a piggy gets a cold, their appetite can suffer. Do you weigh your guinea pig weekly? By the time we noticed my Marvin wasnt eating properly due to his cold, he had already lost alot of weight :-\