My son's father keeps claiming him on his taxes without my permission, what can I do??

My son's father has claimed him for the last 2 years without my permission. I want to figure out how I can make sure it doesn't happen again this year. My son lives with me, I support him and take care of him. His father pays support but has no involvement in my son's life. I am the one who gets him to and from the doctors, I make sure that he has food and clean clothes. I make sure he gets to and from school. I would really like to find a way to prevent his father from claiming him.