I started out slowly, and later graduated to a toothbrush.
First of all, I didn't start teeth cleaning with a brush or finger stall, which I think you are referring to. I began introducing Coco to my wet finger in her mouth, gently massaging her gums. No toothpaste.
Next, I used a little piece of gauze wrapped around my index finger, dipped in water. Again, I massaged her gums and tried to brush her teeth with it. Really she was just biting down on it. You are just trying to get her used to having your finger in her mouth. (I did this for a while, but it paid off.)
Then one day, I put a little doggie toothpaste on the gauze, and she went nuts! She loved it. So the next day I took out the toothbrush, but it looked huge for her tiny mouth. I had a baby toothbrush in the house, the kind for a human baby- it had a nice small head. It worked better. She really looks forward to having her teeth brushed and she's very cooperative!
It takes patience and time. I wouldn't rush into using a toothbrush yet. Get him used to having you put your finger in his mouth. I've heard a lot of dogs hate toothbrushing time- so you aren't alone.