I don't know what applies to non-married people but I sure know what applies to married people. You need a lawyer but try to stay out of court. Try to settle. Litigating is so expensive you just wouldn't believe it. So many thousands of dollars, heartache, and misery.
A good custody lawyer will charge about $400+ hour in Northern VA and you will need to put up a retainer of about $7K to hire one. I am speaking, generally.
You can find the top lawyers in the Washingtonian "Best Lawyers" issue that comes out every year; it's at the library. You might consider getting a female lawyer to represent you. It might help.
Keep it in Fairfax County and avoid Arlington if you can. Fairfax County is called the "rocket docket."
The Bar will let you see one lawyer for free or/for $50 if you go through their referral but usually they are not the best ones.
The more research you do, the less you will end up paying a lawyer. There are great books on this at the bookstore (from the dad's point of view).
If she is asking for full custody of your biological son, she prob. won't get it. You will get visitation (like every Wed night and every other weekend, plus 2 weeks in summer and half of all school holidays) and you will need to pay child support based on a formula that takes into account both your incomes, or your imputed incomes, if it comes to that. At least, that is how it would work if you were married.
It is quite straightforward, really. No matter what you have done, almost everyone gets the same deal in the end. You'd have to be an incredibly abusive and rotten dad to get no custody at all.
Be sure you get joint legal custody (that allows you to have equal say in educational and medical decisions). She will get primary residential custody and you will get visitation.
You will be co-parenting for 13 more years. That means constant communication and negotiation. So your best bet is to make nice now as much as you can and repair the burned bridges. Your lawyers can hurt her and you can pay them to hurt her. Don't do that. It will just make the lawyer rich. That is not a win. Take the high road. You may end up paying more but don't forget -- you are a co-parent. Keep it respectful. Years down the road, you will be glad you did.
I'm assuming she is a good mother and you have no issues with that. A child can be happy with two stable homes, but do try to get along with mom as well as you can. She may not be your biggest fan right now but you can always tell her what a fine mother she is and how important the children are to you, and that you will be there to support them. This is what she needs to hear. She knows sons need daddies. She needs to know you will be a good one.
Don't EVER denigrate the mother in front of the child and be prepared to show that you have adult supports and a good home/apartment where the child will have his own space for visitation. A child custody evaluation runs about $25K. Try not to go down that path. Negotiate, settle, and mediate all you can.
The 9 year old son may have bonded with you. Please don't forget about him. Don't panic. Everythig is going to be okay. Try to see the kids as soon as you can. They need to know you are still in the picture. Good luck to you.
but I bet he/she will say the same thing.