My ex wants custody

So im currently 29 weeks pregnant with a little boy. My ex wants to file for full custody of my son because he got caught up in a bunch of lies and turned out to still be married and having an active relationship with his wife while telling me they were getting a divorce and were living separately. Since finding out all of this he has done nothing but put me through insane stress and have been told that im at risk for preterm labor with how stressed ive been. Is there any way possible that he could ever get full custody of my son?

Edit: Im not trying to keep him out of my sons life. But he’s also making it extremely difficult to even come up with a plan to co parent unless i give him what he wants