He could have fleas or allergies. I would take him to the vet for proper diagnosis and treatment. Have you changed his food recently or given him anything different that may have caused allergy, or it could be allergies from the grasses he goes in, pollens, environmental, etc.
Fleas is a possibility. They also seem to be pretty prone to demodex so you may want to check that out. Otherwise there's a strong possibility of allergies. Check your food to be sure it's 5 or 6 star rated (reading the reviews can be quite informative) on the analysis site and if not try and find one you like near you. It makes a WORLD of difference though it can take a bit of time from the slow switch to get all the corn and chemicals and things out of the system it's well worth it.