I also had tough time brushing my son's teeth . Then one day I brought 'pepper series-books' for him.He loved them so much that he wanted to brush his teeth like pepper did.
We sing along. I make him stand in front of mirror & he loves to see bubbles in his mouth.
After he is done its my turn to clean his teeth.
We also change his toothbrush every month. I take him to the market & he can select his own brush.
He now enjoys brushing his teeth.