Hopefully I can make some sense of this.

so for the past 3 years my boyfriend and I have had to file married filing joint. He got audited because he claimed one of my kids as a dependent. A tax advocate said if we are going to claim the kids (biological they are mine, he claims as his step kids) we will have to file married filing joint from here on out. But we are not legally married nor com in law. we have been together for 5 years. Unfortunately we split up and I did give him permission to claim my kids this tax season. Will we have to file together? If we file separate can he claim one of my kids? I know this sounds confusing. But from what the tax advocate told me sounds weird. Please help!