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Q. Does that override the orders for filing? 

A. No. The order is still binding.  But if he decides not to comply, the IRS will side with him. Your only remedy is to go back to court.  You have an additional problem: because the child physically did not live with you for more than half the year, you are not technically entitled to all benefits.  See full write up below.  But, as long as he doesn't try claim anything, you can  get  away with claiming everything.

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 For tax purposes, there is no such thing as joint custody, regardless of what your legal agreement says. The requirement, to be custodial parent, is that the child live with you MORE than 50% of the time. One of you has to be the custodial parent and the other the non-custodial parent. The IRS goes by physical custody, not legal custody. 

In the rare case (could probably only happen in a leap year like 2016), where the time that  each parent has the child is exactly equal, then the parent with the higher income (AGI) is the custodial parent, for the purpose of determining who has first priority on claiming the child as a dependent. But then neither parent can claim a Qualifying Child dependent, for some tax benefits because neither parent had the child the required MORE than half the year. (no earned income credit,  based on that child, and the child would not qualify the parent for Head of Household filing status).

Yes, the IRS expects you to count the numbers of nights the child sleeps at each parent's home.Then you are each the custodial parent of one child. It is allowed  for you to arrange the children's schedules so that the child spends more than half the year with the father one year and more than half with the mother  the next year so that you are each the custodial parent in the year you claim the child, so that you can claim full benefits.

Equal number of nights. If the child lived with each parent for an equal number of nights during the year, the custodial parent is the parent with the higher adjusted gross income (AGI) (Pub 17 pg 28).

The custodial parent has first priority on claiming the children on her taxes; regardless of the amount of support provided by the non-custodial parent. The IRS goes by physical custody, not legal custody. The non-custodial parent can only claim the child as a dependent if the custodial parent gives permission (on form 8332) or if it's spelled out in a pre 2009 divorce decree. (without conditions - usually the payment of child support). Even if a divorce decree, dated after 2008, gives the non-custodial parent the right to claim the child, he must still get form 8332 from the custodial parent. A properly worded decree should require her to provide that form. https://www.irs.gov/pub/irs-pdf/f8332.pdf

There is a way to split the tax benefits. For future negotiations with the other parent (and maybe even for this year) the following info may be of use:
This may be helpful in your negotiations with the ex:

 There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner.

Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child, even when the custodial parent has released the dependency to him.

 So, it's good idea to let the other parent know that you will be claiming those items, as many first time divorced parents are not aware of this rule and may try to claim those items, which will cause the IRS to send out letters.

Ref: https://www.irs.gov/publications/p17#en_US_2017_publink1000170897

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You can if you are the custodial parent.  The custodial  parent is (the parent the child lived with for more than 183 days in 2016.