I haven't read through the other answers so my apologies if this is a duplication...
Our pediatrician said it can be "normal" for a fever to leave and return, when the body is fighting a bacterial infection, not just a typical virus. E.G. a sinus infection vs. the common cold. He said that's why they make you wait so many days before prescribing antibiotics, to ensure it's really an infection.
Anyway, just remember that a fever is the body's way of fighting off the sickness. I went through a similar panic a few months back, when my son had a bacterial sinus infection following some other virus and was really sick for two weeks straight (poor little guy). As long as you're able to keep the fever at 102 or below with Tylenol, he's fine. If it goes higher than that and Tylenol can't control it then you should be concerned. You might also want to try Ibuprofen at night if he's having trouble sleeping, since it works longer. Just make sure your pediatrician agrees on a rotation and don't mix the two medicines during the day.
When my son had the sinus infection he wouldn't eat or drink either and he lost a lot of weight. He was skinny to begin with so it really had an impact. We just kept offering him milk and juice (which he isn't normally allowed to drink so much of) and he drank *just* enough to stay hydrated. After being sick, we just bulked up on whole milk products (cottage cheese, yogurt) and pretty much let him eat anything to regain the weight.
Hang in there mama! If your pediatrician thinks all is well, you're probably fine, but don't ignore your maternal instincts either. It's better to look neurotic vs. worrying yourself silly.