Question

how can he overcome his fear of water when swimming

Answer 1

He will become more scared of water when he tries to learn swimming.The other children who have come to learn swimming will make fun of that child and the trainer will also scold him.
I would tell him that there is nothing to be scared of water.I will ask him/her what happens to him/her when he/she gets into water or when she/he sees water.I will also learn swimming along with him/her to provide support.I will advise that child to have more practice in swimming .This will probably help that child.