It is difficult to b esure about this, but one well recknown cause is a sand worm infection. This is caused by sand being contaminated by faeces from dogs or stray animals. It also has the longer medical name of cutaneous larva mugrans. Look it up on Google and see if the rash is similar to your son's under his feet. he should wear shoes when on the beach especially if it is a beach wher people walk ther dogs. There are medicines such as albendazole which can be used to treat sandworm.
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