What you are describing is a pretty typical condition in cats that is similar to eczema in humans.
Do you feel any rough scabby like areas where he is itching?
Is the area of hair loss just shorter hairs or is he licking those areas completely bald?
He is licking both hair and bald parts.
Thanks for the information. If he is that itchy then almost certainly antihistamines will not work.
Antihistamines are wildly ineffective in cats and bitter, so they are hard to give
What you should consider is having him tested for ringworm
If he tests negative then a trial of prednisolone will make him feel better and treat the most common kind of condition that affects cats like this
The condition is called miliary dermatitis an is an immune system abnormality.
The prednisone stops the over reaction of the immune system and he feels better and stops licking his hair off.
There are scabby patches.
That makes it almost certainly the miliary dermatitis, if he passes his ringworm test he'll likely respond very nicely to prednisolone from the vet.