Thanks for the additional information.
This situation doesn't sound at all odd since trauma of this sort happens all the time! But, it takes a great deal of force to cause a concussion where cats and dogs are concerned so I'm not entirely certain that this is his problem.
But to specifically address your question (on the off chance that it is) as to what a vet might do if a feline patient has a mild concussion, much depends on what the physical exam reveals.
For example, if there's evidence of neurologic deficits (balance issues or if his eyes are abnormal), then mannitol might be given to reduce edema in the brain.
If the patient isn't eating or drinking, then intravenous fluids would be given for stabilization of blood pressure.
In some cases, I might not treat the patient with drugs at all since most cats will improve with time.
It's also possible that the trauma has nothing to do with his current symptoms although I'm not a huge fan of coincidence. If you have a rectal thermometer, you can take his temperature. Normal is between 100 and 102.5. If it's elevated, then it's doubtful that his symptoms are related to any trauma.
Other possible explanations include: viral or bacterial infection or Fever of Unknown Origin (which is exactly what it sounds like).
Cats with pancreatitis will sometimes behave as he is doing but they aren't typically running a fever.
Cats can also have "off" days and then revert to normal after 2-3 days at most.
At this point, I would continue to keep him fed and hydrated as best you can. You could offer human baby food (avoid those with onion or garlic in them) or boiled chicken which he may prefer to his regular cat food.
Or you could mix canned food with liquid (the milk replacer, low sodium chicken broth, tuna or clam juice) in a blender and use an eye dropper or small turkey baster to dribble this mixture into the side of his mouth.
I hope this helps and that he improves within a very short period of time. Deb