In California, if the parents cannot agree on a custody arrangment, they live close enough, and they are both pretty good, custody is usually split down the middle 50/50.
Also, once the children have been living in an invironment for an extended period of time and started school in that invironment, the court is very weary of changing it unless there is extinuating circumstances that warrant it.
My husband's ex beats on her husband in front of the kids. She has alcohol parties in front of the kids. She aslo does everything she can to emotionally alienate the children from my husband and especially myself. With all that said, she continues to keep the 50/50 custody.
It's going to be very difficult for you to explain why you left an abusive husband, took one child, but left the other behind voluntarily and now want that child back.
You will definitely need to get legal advice from someone with experience in this. Don't just take the first one or get one based on his rates.
The judge may say he agrees to keep the children together, but not necessarily in just one home. He may split it down the middle. Or keep them in the home that will do less in disrupting their lives. Since the boy is in school, he will probably lean towards keeping them in that area. You may have to relocate.
If I were you, I would do my homework and get as much evidence as I can. You have a right to speak to the teacher & school staff. Show some concern and give them your number if you haven't already done so.
Also, just because the court gives you what you want doesn't mean it will stay that way forever. My husband received 50/50 custody in 2002. His ex has taken him to court every year since then to try to take them from him. It's expensive and stressful. She never gets her way, but it doesn't stop her from trying every year.