With him being in arrears, is he still allowed to claim 2 children this year as stated in our divorce decree?

According to my divorce decree, it states that my ex and I, the noncustodial parent (the father) are to alternate tax years filing the children. We have three children. It states that until their graduation from high school, one year, I will claim two, and him one child, and the following year, him two children, and myself one child. Last year I claimed 2 children, and it is his year to claim 2 this year for (2014). I had it set up where my support is monitored by the state. He is currently in arrears $1700 for past due child support. Unfortunately, the attorney did not word the divorce decree to state if he was behind on support, he could not claim the children. I however, think it's unfair, that he will get a larger return, when his children have been put on hold for things they need.