There may not be much that you can do at home for this. If he is not neutered, many male cats have very heavy oil secretions on the tail and sometimes at the tail base that can set off secondary pyoderma. But often the scabbing over the lower back and tail is a pyoderma (deep skin infection) usually caused by flea bite dermatitis. Not seeing fleas does not rule this out, and many times even if the fleas were recently eradicated by Advantage or Frontline or other effective control, the infection may have already set in. He will likely need several weeks of antibiotics and medicated shampoos but also may need some diagnostics like skin scrapings and possibly fungal cultures to rule out other problems. But this is all up to your regular veterinarian. Please go to him or her and have a good thorough exam when you can.
Dr. Jay Mawhinney
Sometimes the irritation that sets off the over grooming and then secondary infection will be from other allergies. Something in the house could be causing it or outside allergies. Since it is seasonal by your description. However he may need skin scrapings etc to rule out underlying parasites. More likely it is pyoderma from some over grooming from allergies. Again fleas are top of list but not only possibility. Food allergies for instance usually cause cats to scratch their faces.