Thanks everyone for your help, we're going to check with his doctor if it could be the sensory disorder, he does also seem to have a problem with food textures. We're also going to keep trying the swimming lessons and swim parties with his friends to see if he would be more motivated in that environment as well.
If he doesn't have the sensory issues as others said, he may need more time/contacts with water. First, try to encourage him a lot, give a small reward if he would like to try. My 5-year old suddenly could put his head and face in shower about 1 year ago after lots of encouragements and rewards. We set a goal for him: by his 4th birthday he should be able to do that. He did it before his birthday. But he wouldn't like to put his head in water when swimming until last month, when his daycare went to a swim lesson together. I guess when he saw his peers/close friends could do that, he would like to try it too. So I'd suggest to take him to the pool more often, maybe better with his friends who can demonstrate that. Good luck!
I have seen many children do this they cannot catch their breath I like Annie's idea of washing his hair ,the swim instructor should be able to work on it with him, in my opinion its not unusual ..good luck
I agree with the others on the sensory issues - sounds like tactile defensiveness to me too.
Does he have other tactile issues - tags on clothing, socks that don't feel right, food textures?
He may have a slight sonsory disorder you can see an ocupational therapist to have some desensitising. Hope this helps
I don't know about the swimming, but if you want fewer tears at bathtime, try this: lather the hair, then get ready for rinsing. Have him hold a folded-up towel over his face and ears. Either tip his head back so he can put it under the faucet, or so you can pour water from a plastic pitcher onto his head. You pour and rinse as quickly as possible. If you work fast, you can get the hair rinsed faster than the towel gets saturated. Much less creepy or scary for a kid who doesn't like the sensation of the water on the face.
So google this and see what you think.
Some kids are just afraid of water as well---------- but check into sensory to see. An 'over reaction' like that is often explained for some unforseen reason. goodluck