My Husband Has A Hard Sore Spot On The Side

My husband has a hard sore spot on the side of his left arm... there was some white stuff that came out of it about a year ago .... he said it felt like something was still in there and it was white and hard .... and all at once its flared up again ,and its almost as big as a fifty cent piece. He thinks he could have hit it on something when he was taking down a tree stand the other day
What could it be and how can we doctor it at home??