When he was a baby that time, I use handkercheif to clean his mouth. Starting at 2 years old, I already trained my son to brush his teeth. I took a small basin into his room together with his cup & toothbrush and my one. And we brushed tooth together. And after let him to put it back all the thing one by one. He felt very happy and very confidence. He seems he is a big boy.
Now, he is two and half years old, when I said 'Brush Teeth at a night time, I will follow me to go to toilet and brushes teeth with me.
I hope you do it with him all the time, may be he will listen and follow to you on everything.