Non custodial parents wants to claim 20 year old

Tax expert advice if anyone knows here: Can a non custodial parent still claim his 20 year old as a dependent? He contributes nothing financially, rarely if ever sees him, HOWEVER, divorce decree says he can claim him as dependent for as long as he is eligible to be claimed as a dependent.

From what I have read from IRS website is he cannot unless other parent releases a tax form allowing him to. However non custodial parent is claiming rights to claim him until 24 because of decree.