V.B.
My daughter did this at one point, but I realized it happened right after we had visited my Mom's house that didn't have a mat in the bottom of the tub. She slipped a little and after that was terrified of the bath, even at home where she had a mat. I would suggest getting one (if you don't have one already) and then you may just have to sit in there with him until he gets over it. I had to do it a few times. I also sang to my daughter and played little games with her to distract her. That seemed to work well too. Within a week or two, she was back to normal.
Good luck!