Welcome to parenting. This is one of the hardest parts, when you child is sick, and you are doing all you can, and you feel SO helpless.
Even if your son has a virus, the amoxycillin will help with the symptoms, because for some reason, antibiotics do that. And he should show signs of improvement about 24 hours after starting the medication. (Sometimes docs prescribe the drug for a viral thing, as a precaution, as well, to prevent a bacterial infecton getting started while the immune system is overloaded by fighting of the viral infection.)
How often are you giving him motrin ? the drug should work for 4 to 6 hours, so he should be getting at least 2 hours of slightly better times out of every 4 or 6. And make the most of those times to push fluids and whatever foods he wants.
Generally, when my kids were sick, I fed them just about whatever they wanted. If they thought it sounded good to eat, I figured it meant they felt good enough to eat it. And if they are asking to eat, why make an issue out of what goes in ?
If you are concerned about dehydration, and he is hungry, you can offer juice (a small amount -- like a shotglass size) and tell him if he drinks that juice, then he can have what he asked for. That way he gets more fluid in, and he gets what he wanted, too. Water is okay, too, remember.
He'll probably be improving by the time you get done reading all the posts you get back -- but if he doesn't show improvement after 48 hours on the antibiotic, you might want to call the doctor back. Or, if he gets listless, hot and dry, (dehydration), DEFINATELY call, and expect a trip to the ER so they can run saline in thru an IV. That latter one is kind of a worst case scenario, so don't get overly worried about it, just know that if he gets too dehydrated, there is a solution, and there is a support system in the medical community to protect him. (You can also watch his urine output to check on hydration -- the clearer his urine is, the more fluid he has in his sytem. the darker it is, the more dense, and he needs more fluid.)
Hang in, Mom ! You'r doing everything you can. And see if you can get a little rest while Dad is home, (Maybe the two of them can watch football together today !) that way you get a nap, and you don't get sick once he is better !! :-)