Thanks for the answers to my questions and the additional information although I'm sorry I was off my computer when you did so.
That's great that he's eating and drinking. It's always to good to double check a cat's temperature in these situations since a fever often explains lethargy. If you manage to take it, normal is between 100 and 102.5.
I always worry about a urinary blockage when male cats are growling and don't want to be touched but if he's not particularly overly sensitive when you palpate his belly in between his legs, then this condition is less likely. Or if you've seen him urinate within the last few hours, then he's not likely to be blocked.
Given your description of his behavior, several thoughts come to mind:
1. He could have been in a fight or encountered another critter and been injured although there are no obvious wounds on him. Abscesses can sometimes take a day or so to develop and become obvious.
If he lets you without getting injured, I'd gently palpate over his entire body very slowly to see if you can isolate one particular area of pain or discomfort.
2. He might not have been bit but perhaps he was involved in some other trauma which has caused bruising or soft tissue injuries to muscles. You're not likely to find specific evidence of either so this is sometimes a diagnosis after ruling out other things.
3. Another possible explanation would be Toxoplasmosis which is a condition commonly contracted by ingesting rodents (mice and such) or birds. Since he's an outside cat, this condition would have to be included on the list.
Many of these patients will run a fever although not all of them will but they can also experience balance issues or discomfort.
Given his outdoor lifestyle, the most common explanation is going to be either #1 or #2 above.
I'd dearly love to be able to suggest over the counter medications for you to give him but since cats are highly sensitive to so many of them, it wouldn't be safe to do so at this point without knowing what exactly is causing his behavior.
If you're able to localize a specific area of discomfort, you can apply a cool compress to the area which may help.
Ensure that he's urinating and continue to encourage fluids, which will be important if he's running a fever.
If he continues to behave this way tomorrow, then a vet visit may be prudent. I've seen a few cats just have "off" days, but I doubt that this is his issue. It sounds as if he's in a degree of pain or discomfort...the source of which may not be immediately apparent but is present nonetheless.
I hope this helps to provide conditions which might explain why his behavior has changed to dramatically in such a short period of time. Deb