There are lots of viruses and infections that don't involve upper respiratory issues (runny nose, throat problems, coughing). My concern is that he is not drinking fluids. He really needs to. Water, herbal tea (no caffeine), broth/soup, even pedialyte, and stuff like chicken soup if he'll do it. It can help lower the temp as well as flush out whatever virus or infection is bothering him. Food is not as important, and sometimes when someone is under the weather they just don't have an appetite. But you must make him drink fluids.
You have quite a few posts about being worried and anxious about your sick kids. I think, when you see your pediatrician, you should get a really good list of things that require a doctor's visit or consultation, and things that don't. Also get a guideline of when you do something to break the fever, and when you don't. A fever is a sign of the body's immune system working, so you don't usually medicate it unless it's extremely high (102 or more), or if the child has a lot of aches (body aches, headaches, sore throat). Motrin and Tylenol don't fix the problem, so yes, the fever will go back up when the medication wears off. That's normal. If you do take him to see the doctor, avoid giving him any medications for 4-6 hours beforehand. You want the doctor to see the child the way he really is when he's sick.