Tax returns

My ex husband and I have 50/50 custody of our 2 year old.

He claims our son on his taxes, I don’t. This is our first year filing divorced.

On my return, it asked if I have kids. I said yes. Then I put in his name, birthday, social and selected that the other parent claims him. It then said I do not qualify for him as a dependent which is what I wanted.

My return was filed and accepted.

On my finalized return it shows I have no dependents.

My ex husband’s return was rejected because our son’s social security number was used on another return, MINE.

But I didn’t claim him so I don’t understand why it won’t let my ex claim him.

My ex is now saying that his return went from a couple thousand to $120 and that I owe him half of my tax return because of it.

I’m confused and irritated.

I’m pregnant with my & my boyfriend’s child and I’m due in April AND we’re trying to get into a place since we don’t currently live together, so I really can’t afford to give him half of my return!

Why would it not let my ex claim him???