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Your son have developed a bruise over your skin due to injury
A bruise develops due to internal tear in blood vessels and bleeding.
Just apply thrombophobe or hirudoid cream over the affected area.
Thrombophob ointment contains Heparin-Benzyl Nicotinate which improves the blood supply and prevents clots, so it will help in faster clearing of the clotted blood.
Since he seems to be developing bruise at the same place again and again , you need to find out the cause of recurrent trauma at the same site again and again.
Hope it helps
Dr Geetika Paul