Something that really helped at our house is to use the vapor solution that you can buy to put in the humidifier. It really helped both my daughter and I. You can try giving motrin instead of tylenol and see if that helps. The anti-inflammatory action of the motrin may be what he needs, especially if his sinus membranes are swollen (and they probably are). Remember, if his mucus is green or yellow, that is a sign of infection (like a sinus infection for instance) and it is no longer just a cold anymore. Use saline solution to irrigate his little nose 3 times a day it will really help, along with using a nasal aspirator to pull whatever you can from his nose..eventhough he will probably hate it, its best.
You know, that rash scares me a little, I am surprised they didnt say more about that at your doctors visit. If he is getting worse not better in a day or two, you need a second opinion. Look for additional syptoms like fever, struggling to breath, green/yellow mucus etc and take him in asap if any of those happen. Good luck!!