Have you taken him to the doctor? I would do that, jic its an ear infection or something like that?

My daughter had a UTI recently and one day we went to have it checked & there wasn't anything obvious, the next day she had a febrile convulsion and was a very ill chicken. So maybe just bring him to the GP. At hospital, they would administer panadol or neurophen if the temperature went over 38. If it was under 38, they didn't worry.

ETA: I would say over 24 hours and its time to get it checked out. It doesn't necessarily mean that antibiotics will help, its more likely that its viral and antibiotics won't help, but its worth getting checked to make sure.