Claiming son on taxes

So my son is 5 years old and his father and I are not together. He’s claimed our son every year since he was born since he had health insurance for him. I have dental insurance. We kinda made a deal whomever had the health insurance could claim him. In October I’m adding him to mine since I have two other children with my husband and costs nothing to add another. My ex says “hell no” to me claiming him this coming (next) year. What are my options? I feel like he is being very immature about it. He said I have two other kids to claim which is true but that’s no concern to him. I know he will file and claim our son before I ever get a chance too but didntknow if there’s any laws over this or if this is something to go to court beforehand for? Anyone have advice? I’d rather not go to court as I don’t have the extra money (if it costs money for court?) but I will if I have too.

Also crossed my mind to have him carry the dental insurance as well if I cannot claim him.

We share 50/50 custody. Although most his time with his father is spent with grandma but I won’t get into that.