Normal is between 100 and 102. 5 so a fever is not responsible for his symptoms.
There are likely to be two possible reasons for his behavior
1. An issue with the spine such as compression by a disc or mass. Not all dogs will react with pain when palpated but they often walk very slowly and hunched over. His balance could also be affected.
X-rays might be suggestive of this problem but usually an MRI is needed.
2. Tick diseases can cause these symptoms. While many dogs will have fevers, some of them don't.
These conditions can occur quite suddenly with no warning. These could be ticks that fed months ago (lyme
disease) or one week ago (Rocky Mountain Spotted Fever). We do have tests for many of these diseases, but we are testing for antibodies, not the organism itself, in most cases. If antibodies are not being produced (for whatever reason) then the tests could be negative and yet this is still the underlying problem.
I've also come to believe that there are tick diseases that we haven't even been able to identify yet and thus would not have tests for all of them. Doxycycline is the drug used to treat most tick diseases.
3. I'm assuming that he doesn't have access to drugs or medications although these would have to be included on the list as would mold or other toxic
substances that he might have ingested.
If you wanted to watch him for the rest of the day, especially since he seems a little better, then this would probably be fine. You could give him Aspirin at a dose of 325 mg 1.5 tablets twice a day (with food to avoid stomach
upset). This drug has anti-inflammatory as well as anti-pain properties which might make him feel better.
My only reservation is suggesting it would be that it might interfere with what your vet would want to prescribe (if you have to have him seen). But one or two doses should be fine.
I hope this helps and that he's feeling better soon.