My Son Use's A Shampoo For His Hair Called Ketoconazole,

my son use's a shampoo for his hair called Ketoconazole, is there any over the counter shampoo he can use that will work because I have to call the doctor to get refills on this shampoo and he goes through it in a month and half and once he stops using it with in a week he has big thick patch of dandruff and flakes