I will try to help you with this, but first let me ask a couple of questions:
Let me know and I will continue to advise you.
It does help quite a bit....rules out a lot of the things that might have been more serious. I agree with you that it is very possible he could have been bitten by an insect. Cats are very curious, and many indoor cats are voracious bug hunters!
The only way to take his temperature is to use a digital thermometer rectally. Normal range is 100-102.5.
This is too far down the road to be a vaccine reaction, so I wouldn't worry about that. What I would try is giving his some antihistamine to see if he improves....if it is a bug bite, he should...if he doesn't improve, I would not hesitate to take him in tomorrow for an exam. It is possible he could have an ear infection or a tiny foreign body down in his ear that can be seen with an otoscope.
For tonight you could give either benadryl or zyrtec...which one do you have on hand (if either) and how much does he weigh...if you are comfortable giving him a pill, I can advise you on dosage.
Oh, capsules are tough! He would take 12 mg....which if you had tablets would be simple, but capsules are a bit tougher to divide the contents easily. If you have a convenience store near by I would go get tablets....then he would take half of a 25 mg tablet every 8 hours.
Hope that helps....and hope this odd episode passes on its own. You sound like very attentive and concerned owners!
Best wishes!
Veterinarian
9 years of experience as a companion animal veterinarian with emphasis on nutrition and senior care.