Does the powder you're using kill mites too? Dogs generally acquire mites in their ears, not ticks. If you see a buildup of dark brown waxy material in his ear, it is mites and you will need to get an appropriate solution/drop to put in his ears to kill them.
You shouldn't have to clean your dogs ears that often. I clean my dogs ears 3 times a year (natural eared doberman), that is all he requires as his ears stay clean on their own.
If his ears are clean, and he is still shaking his head/itching, he could have a neurological problem and a trip to the vet would be absolutely necessary!