Dog Allergies Treatment

My dog, Winter, scratches all the time. He doesn’t have fleas. I believe he may be getting bit by ants or he may have skin allergies. I hate seeing him like this. And recently it’s getting worse. Unfortunately, the vet can’t see him until next week. Can you answer these questions for me?
What is the proper dog food to by for dogs with itchy skin?
What is the proper shampoo for dogs with itchy skin?
How often should I bathe him?
-I was doing once every two weeks is that too much or too less?
What else should I be doing until his appointment?