My son gets a rash every time he swim at the sea or at swimming pools

I have heard of children who react that way to the sun. Maybe your son has developed a allergic reaction to the sun. I don't know anything else about it it's just something I've heard before. And I don't think applying sunscreen helps the problem.

Common rashes include eczema, poison ivy, hives, and athlete's foot. Infections that cause rashes may be fungal, bacterial, parasitic, or viral. One of the best come over with rashes is consuming camel milk. It works amazingly in the case of rashes and other allergies.

You can also read about it here

https://www.veryrareonline.com/2017/11/02/camel-milk-food-allergies/

I don't know if it's an allergy or not but I have a lot of allergies as well as sensitive skin and when I go swimming in the ocean or some pools, I get incredibly red and itchy as well. If he was only tested for food allergies, go back and ask for a more extensive allergy test. I assume you were at an actual allergist? If not please see one, I assume not all GPs are well versed in allergies. 

I always thought it was just because my skin is sensitive and the salt water or high chlorine levels irritated it. Maybe it is an allergy, I've never looked into it. There are several conditions that come up when you google "rash after swimming" so I would look into those. Good luck, and I hope your son feels better! 

It is quite clear to me that he is "allergic" for salt

The thing is that skin is a protective layer and when it's weak, like when a person has an atopic dermatitis, it's not protecting your son. You should be careful with salt at all. As well, please use medical cremes/ body lotions that are used for atopic dermatitis

I don't know the exact reason, but I can relate it to the situation that my friend Loosie's son came across. When they moved to Boston they selected a house with swimming pool. Her son is interested in swimming. But it needs some repairings. They found an instructor and taught him swimming. They are getting hard water in that locality. After swimming in hard water pool he had rashes and skin allergies. Loosie was tensed and shared her worries with me. I told her to consult a doctor. After the tests doctor told it can be caused by food allergies. But he had the issues only when he swims in the pool. Loosie's neighbor told that her son too had this issue and it was because of the hard water supplies. Hard water contains minerals that dry out the skin, clog pores and leads to stubborn skin conditions like dermatitis, acne, and eczema. When you hire a swimming pool repair service( http://www.ferraripools.com/pool-service-maintenance/ ) from your locality, make sure you check the quality of pool water.  

Did you ever figure out what is causing it? I know that this happened to me randomly one summer and it turned out that I'm allergic to the cold.