Fear of water could be said to be one of the more natural fears, or phobias - like fear of heights, yet while natural these phobias still need to be treated with respect.
People often ask what causes a fear of water, but there is no single answer - human beings can develop fears of anything - from buttons to balloons (we have seen them all), but the good news is that.... any fear / phobia is a learned response, therefore it can be 'unlearned' - YOU need to re-educate your sons unconscious mind to relax around water.
Try asking your son what he THINKS will happen to him when he is in the water.... what does his child like (wild) imagination see (and predict) when he feels the water being poured over his head?
After you have learned all about his thought process behind the matter, take some time and discuss it with him, he needs to be redirected to the TRUTH about water - let him know that water is not unpleasant or scary.... that water can actually be his friend.
Try giving him goggles to wear and see if this item seems to calm him while in the water.
Good Luck....
LoVe,
Rhapsody -