Claiming form??

Writing from Wisconsin, it’s 3:30am as I write this and I’m not thinking very clearly to remember a tax form name. Back in May, I moved into my own apartment separate from my partner, we have a child together. We’re still together but needed to separate for a bit. We’re not married. Each year, he has claimed our son but this year, the custody is 50/50 and he pays me $230 a month in child support. Each year, we’ve heavily depended on the tax return for our son’s future savings and debt we’d acquired in the past. I do not want any of this to change. Is it still possible for him to claim him? I believe I heard there is a form that we sign but I am the one that does the taxes, self service. What do I need to look out for and is it going to hurt the amount by him having to pay child support for 8 months? I probably sound stupid but I want to be prepared. Apologies in advance if this isn’t making sense, I’m exhausted 😴