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If it is primarily only on the side that he predominately lays on...then contact allergy is a definate rule out. Carpet freshener and cleaner are big possibilities...
Try not to eliminate the cleaner, bathe him and see if it clears...then if you clean carpet again and it returns...you have your answer.
Look at the bottom of all four feet, many times they become irriated with carpet cleaners too.
If you are sure that he is clean of fleas...you can purchase hydrocortisone sprays/cream or benadryl sprays/creams either or both can be used. Benadryl is an antihistamine and cortisone is a steroid. They both will help...hopefully you will be able to handle this without medications. Cool baths with oatmeal based rinses are good too.
Great, let me know how it goes!