As a pediatrician, the decreased drinking also has me concerned. If he can still manage enough sips to urinate well at least once every 8 hours (e.g., 1-2 ounces intake every 10-15 minutes), then dehydration has probably not developed in the setting of presumed viral illness with fever. If he actually goes for more than 8 hours without urination, then call the pediatrician right away or consider an ER visit since intravenous (IV) fluid rehydration may be needed. If the difficulty with drinking is due to nasal congestion, then consider the following: a cool mist humidifier (avoid a hot/steam vaporizer as this may worsen nasal congestion) near the bedside; pre-feed treatment with nasal saline 10-30 seconds before short-tipped bulb suction (the drying effect of the saline may help to open swollen nasal passages).
The ER visit sounds wise, even if just to make sure he is or is not dehydrated. Keep coaxing small sips in the meantime--in fact, if there is nasal congestion, it may be easier to feed from a regular cup instead of requiring suction from a bottle. Count the number of wet diapers in the 8-12 hours prior to the ER visit and try to recall how wet they were (e.g., only moist, fairly wet, quite soaked) since the hospital staff will need this history.