Boston Terrier with skin issues...Please help!!?

Hi,I have a 1.5 year old purebred boston terrier.The last 2 months he has been having skin issues...I took him to my vet they ruled out mange.And put him on a special diet.this is not helping.I have been doing some research and found that some ppl add flaxseed oil's or fish oil's to there dog food.I am thinking of changing his dog food to a Lamb and apple/rice formula.Can anyone help? he has been back to the vet twice.I dont allow him to have and dogs treats.he eats only his Medic cal dog food.Any suggestions? thanks