Filing separate

I’m trying to make this seem understanding as much as possible. 
I’ve been in a relationship with the same person for 5 years. We are not engaged or legally married. I have 3 children from a previous relationship. So one year my boyfriend claimed on of the kids and got audited. So since then we have been filing married filing joint. Here’s the thing we are not com in law or legally married. We split up and since he’s been helping raise my kids. Do we have file separate? And can he still claim on of my kids? I gave the okay. We broke up in November. I’m not sure what to heck to do