How could I stop my wife from leaving with child custody of my son?

First of all, beg or borrow the money for a lawyer. They know how to deal with the complexities of law and the courts. Your state may have a statute that prohibits a spouse from moving a child out of state (or beyond a certain distance) without the consent of the other parent or an order of the Court. And usually such laws require a formal notice sent to you by certified mail at least 60 days before the move, and give you the opportunity to object within, say, 20 days. If you absolutely cannot hire a lawyer, and no entity that provides legal services to the poor will help you, go to the Clerk of the Circuit or Family Court: some states have forms you can use. Obviously you must move fast.

If your divorce has been filed, it should have provisions that she can't leave the jurisdiction with the children. If not, you need to file immediately. Many courthouses and local communities have self help clinics.

If a divorce petition is already filed then there is a statutory automatic restraining order in effect that prevent either parent from removing the child from the state. Remind the moving parent that this restraining order exists and that the child needs to be left in the state unless the court has ordered otherwise. If you need a court order for custody, you can go to the self help center at the courthouse; there is also self help on the court's website.

Go to the self help center of the family law court. Get your paperwork started, get it filed, and get her served. Once you get her served (before she leaves the state), she cannot leave with the children without a court order or agreement from you. Get this completed before she leaves the state. If you have already started the case, it should include (in the summons) automatic restraining orders that go into place after the case is filed and served, including that neither parent may remove the children from the state without a court order or written permission of the other parent (see your Summons).