Hi, I can help you with your question today.
Poor little Peter!
Did your vet actually spot a live flea or flea dirt on him during the appointment?
If so, it is certainly possible that he has a flea allergy.
The other cats in the home could be exposed to fleas, but if they don't have an allergy, you won't see the same severe itching.
A single flea bite can cause significant itching, sometimes to the point of damaging the skin.
Aside from a flea allergy, another possibility would be a food allergy.
Unfortunately, there is no good test for a food allergy. We simply change the food to a hypoallergenic diet.
This has to be very strict for a full 12-16 weeks.
With 4 cats total, it would probably be easiest just to switch all of the cats over.
Prior to a diet change, we could also simply try an oral steroid like prednisolone.
If he has flea allergies, he should feel much better right away.
If you have more questions or if I can help in any other way, please do not hesitate to ask. I will be happy to continue further and do everything I can to provide you with the service you seek.