Here is what you do. He is using the old divorce decree as you said, which is no longer valid.
You submit an ameded tax return, form1040X, along with a cover letter describing why you are entitled to the deduction and not your husband. Make sure you point out the decree he is using is superceded by a more recent court order.
Include the following.
1. Front and signature page from the original decree, with only the pages addressing taxes and custody. Makes sure you put a cover sheet on this clearly labeling it as "last names of both of you, divorce decree, OLD and superceded, dated XXXXXX.
2. Front and signature page of the new court documetns with only the pages addressing taxes and custody. If the new documents have wrods to the effect that the previous one is null and void, include those pages as well. Put a cover page identifying this in the same way as before with NEW and Eneforce.
3. Any othe documentation that shows you are the custodial parent:
Make sure you include a statement that you have never signed any forms or statements allowing the non-custodial parent to claim the child.
Then mail it to the IRS. Talking to them on the phone will not get action. You have to do it in writing.