We Went To The Beach - Emerald Isle And My

We went to the beach - Emerald Isle and my son spent the day in the ocean. It was day 5 and he had been in each day before that. He is very light complected so he wears a swim shirt. That day the waves were particularly rough. When he got home and took off his shirt he had blister/pimple like sores under his arms onto his chest only on one side. It is now Sunday and they haven't gotten much better. They itch sometimes but they are also sore and are irritated when he wears a shirt. Any ideas?????