Could he be allergic to something? Has he tried living in a different environment for some time?

What doctor did you see? Normal doctors or specialist?

Allergies can arise anytime. Not necessarily born with it. And it doesn't have to be food allergy.

I wasn't born with a water allergy, but now I get rashes everytime I take bath or sweat or come in contact with water.

Admit the boy to a hospital and get a good specialist to monitor him. Do some test and try to find out what's the root cause behind it. I guess that's what you can do before addressing the problem.