My DS (two), does not enjoying nursing when he has a cold... it's just too much work for him. He gets frustrated and would rather just sip water from a cup--he's always been this way during a cold... had his first at abut 6 months or so.
Another reason (also cold related) is post nasal drip. Sometimes they get so much going down their throats into their tummies that they simply don't have the appetite they usually do. Lastly, think about how you feel when you have a fever--are you hungry? Usually not for most...
Is he on Amox for something specific? Is there some secondary infection that has made this necessary? If so, I have found that the antibiotics usually kick in in about 48 hours or so. Once your DS is a little clearer and the fever is down, I imagine he'll more than make up for lost time on the nursing... my DS goes crazy for a few days once he's feeling better... making up the calories. Generally speaking, babies are the best judges for how much they need. If you are concerned beyond that for any reason, don't hesitate to call the doc or nurse for reassurance.
In the meantime do what you can to keep up his fluids (water from a cup with your help if necessary, or something like pediacare if the doc recommends it--keeps up the electrolights along with fluids).
Hang in there mom--colds are no fun and for the nursing baby and mommy can be especially frustrating. Hope your little guy is feeling better soon.