Dear, regarding a lump under the arm pit with your son if you have noticed incidentally but he having not felt or complained to you, it could be an reaction of lymph system to some minor infection in the vast area it is connected in the body and a full course of suitable antibiotic suggested already should make it melt like ice. But in the event it persists or assume bigger size while you monitor calls for surgeons hand to decide whether it was indeed grew with age and preexisted even before childbirth as cyst of benign fatty collecive and needs to be surgically treated to obviate problems later though today it is no problem.But I assure you this is no serious issue but of more preventive step.