Dry skin?

Hey guys
Got a little question for you. Chester has been experiencing dry skin for about a year now. He is a tuxedo so you can see the skin flakes on his fur. He does not scratch terribly hard, but is sensitive around where his tail meets his bottom if you scratch for him (the most dry area). I have talked to the vet a few times about this. She suggested:
A humidifier, which helps with the static and some extra itching he had
and A food allergy, but I deleted the new food which was mixed in with his old stuff for 8 months with no changes.

He's eating wet food twice a day and Natural Balance/hills dental mixed in.

Thoughts?

Can you bathe him? Maybe a skin calming wash would help. Have you asked about natural remedies? Maybe more omega3 oil something?

Both foods have multiple grains and all but one are not the same grains ... I would suspect further allergies ... the omega levels are likely decent enough for most cats but some might need more ( discuss with vet prior to supplementing ) .. Have the anal glands been checked?? the SD has a bit more fiber than the ave and could be a bit much