if it's filled with fluid then it will drain on its own. you just gotta give it time. the blister on my dog's neck took about 2 and half weeks.
if it's a solid mass, and you think he got it from the scrap, then it's probably scar tissue. possibly a keloid (overgrowth of scar tissue). still nothing to worry about. but it won't go away.
in the VERY RARE occurrence of a tumor it would be removed like any other tumor mass.
~ jo and Gotti Boi (aka Thunder Thighs, aka Jughead, aka The Doo Doo Monster, aka Sexual Chocolate, aka Mister Smiley, aka Sofa King)