I see you've gotten a bunch of suggestions - but another couldn't hurt! From the first bath of her life at 2-3 weeks to her 1st birthday, my daughter was TERRIFIED of bath time in the sink, baby tub, real tub - and also afraid of ocean, pools, etc. At other suggestions, we tried everything, but two things really seemed to help.
1. At non bath time, we placed bowls and buckets of varying sizes around the kitchen and let her splash around, just to get her comfortable sticking her hands in water.
2. My husband or I would get in the tub (WITHOUT her)with toys and bubbles and bathe. She would stand at the edge and watch us, play with us, but not get in. At around the 5th time, she decided she was comfortable to get in our laps, barely touching the water. We would then slowly ease her into the water and stop when she became uncomfortable.
And as everyone else said, this is a phase. He'll likely grow out of it, and soon. Good luck!