The divorce court order states that I claim son in even years (like 2018) and ex-wife claims son in odd years.
Son was emancipated in April 2018, when he graduated from high school. His is a full time student. US citizen.
Son spent more overnights with ex-wife than me in 2018.
Can I claim son in 2018? I know I don't get head of household, but do the overnights actually overrule a court order to claim child exemption?
No, you can not claim your son in 2018.
The special rule for a noncustodial parent (you) to claim a child only applies if the child is considered as living with either of you for at least 1/2 of the year.
However, when a child is emancipated, for this purpose (the special rule for divorces) he is no longer considered as living with either parent after his emancipation.
Because he was emancipated in April, he is not considered as living with either parent for more than 1/2 of they year (at least for this purpose), the special rule for a noncustodial parent (you) to claim the child no longer applies.
https://www.irs.gov/publications/p501#en_US_2018_publink1000226258