Try a new color, or maybe electric toothbrush. Mouthwash can be essential for people who aren't able to clean their oral cavity.
If he refuses permit you brush his teeth, take him to the veterinarian and let her do it for one. Brush your teeth twice each day, morning and night. http://web011.dmonster.kr/bbs/board.php?bo_table=b0501&wr_id=2259874