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It seems to be allergic reaction. Continue to take benadryl until the swelling subsides. You don't need to worry about it so much. But check what he ate, which could be the cause.
The possible causes are medications (penicillin, aspirin, ibuprofen (Advil, Motrin IB, others), naproxen (Aleve) and blood pressure medications), pollen, animal dander, latex and insect stings, immune system disorders such as lupus, some types of cancer such as lymphoma, certain thyroid conditions, and infections with bacteria or viruses such as hepatitis, HIV, cytomegalovirus or Epstein-Barr virus, heat, cold, sunlight, water, pressure on the skin, emotional stress and exercise. You should check what possibly caused the allergic reaction. If it frequently occurs, you should take him to his doctor.