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If there is no blackish discolouration of the skin associated with visible swelling then he can be observed. If they are present then he will need to get the haematoma
drained to relieve obstruction to urination. You can apply antibiotic ointment to the bruising if it is superficial. You can use cool water which will not hurt him much.If any injury
to testis or if he is peeing blood he needs to be seen by the Surgeon to evaluate possible damage. Next time he can wear a groin guard while playing such sports.