I have personal experience with a judge who gave primary custody of my child to the (abusive) father who wanted nothing to do with him (said father filed for sole custody to protect himself from having to pay child support.) I suffered in many of the ways OP has suffered (no financial independence, not allowed to go anywhere [except with his family,] never went anywhere without the child, couldn't ask for help unless I wanted an argument) but the judge saw I had no job and was in a DV shelter, and suffered from a medical condition (that child's father did not allow me to treat) and gave him primary custody. I only get to see our child two days a week.

I'm not trying to scare OP, but I don't want her going into this mess thinking that there is absolutely no way things can't go another direction. If her SO feels his money may be threatened, he may do whatever it takes to keep it secure. She needs to be prepared. Abusers who are good at manipulating can manipulate any number of people, including judges.

I suggest she reaches out to a DV shelter. Many of them have attorneys that work specifically in DV cases and can help her.

OP, document everything. Try to keep communication limited to text messaging, if you can. Leaving will be hard, but it will be what's best for you and your child. Please do not stay. Get out safely, don't go back to him for any reason.