Can a court order a custodial parent to let the non-custodial parent claim the child?

Me and my child's father are in the process of making a custody agreement. The child has lived with me all his life (he's 5). The father has never lived with us. His lawyer drew up papers that state that we will have joint legal custody. I will have primary custody. But it also says in there that we will take turns claiming the child on alternate years. He barely pays $250/month in child support. I am a single mother going to school, and work part-time. I don't want to agree to this and will not sign it. From what I understand, he can't claim the child unless I agree. But if he took me to court, can a judge order we take turns with the exemption? I thought the IRS web page says that the Federal government supersedes state courts. Please help!