I was divorced in 2009. in my divorce decree it states my ex claims the eldest child on taxes. can I claim him since I am the custodial parent?

If your divorce agreement gave your ex the right to claim your eldest child as a dependent, then that will normally control, since you waived the normal rules by signing that agreement. I can't tell from your post whether you told the woman at the IRS that the divorce agreement gave your ex the right to claim the child. It sounds like she gave you the normal rule, which would be the case except the divorce agreement changes it. If the attorney who told you that you couldn't claim your son is a tax attorney and should know what he or she is talking about, I would listen to the attorney. If you ignore the agreement and claim your son as a dependent, and then your ex also claims the child as a dependent in accordance with your agreement, then the IRS is going to get involved. If the IRS comes after your ex, then he will rightly show them the divorce agreement in which you agreed that he could claim the child. That means that you will be the one in violation of your agreement. Unless you can show that the agreement was later modified in a binding manner, so that you would have the right to claim the child, you will be the one who loses that argument.