You might try preparing him by telling ahead a time that it will soon be time for his. Start an hour before and then every 15 minutes, i.e. it will be time for your bath in 1 hour, 45 minutres,and so on. Also a reward system with stickers, or some small reward usually works. And lots of praise for being cooperative. Or if he is cooperative, he can watch a video, tv, or you play a game with him, read a book, etc., after his bath, and tell him you have time to do this because he was cooperative. It may take a while for him to get the connection but eventually he should. Good luck.