How To Prevent Rash After The Surgery?

Yes my son has had cyst removed from his left wrist, and this past August had a tumor removed from his rt wrist both times he had a rash to develope from th wrist to elbow they were red bumps with white heads on them and he said they itched. we are getting ready to have another surgery on rt wrist and was wondering what could be causing this rash and what i can do to try and prevent it this time when he has surgery.