My son is seven years old and yesterday he got like a sting and it has swollen up and water is coming out what is that?

Allergic rxn : If your son was stung yesterday, it may be a reaction to the sting. It is not unusual to see blister formation after a sting, in which a clear fluid may be present. If the fluid is purulent, or appears pus-like, this may indicate an infection and he should be seen. Otherwise, i would keep the area clean, put an antibiotic ot on it , and give him an oral antihistamine to reduce the swelling.