D.H.
Get in the tub with him. I think he is scared.....so Daddy can't put some shorts on or whatever and get in and show him it's safe.....after doing this a couple of times or so, he should be fine. Also, don't put in a lot of water to start with. He'll be OK. Give him some time and he'll come around......this summer, get him an outdoor pool if you can and explain, it's kind of like the bathtub, after you all play in it a couple of times!
Good luck.