I'm going to take my optomist hat off and put my pessirealist hat on.
First, stop trying to control interactions between your kid and the unfortunate mother-to-be. He is going to sleep at her house when he wants to. Its a battle that you have already lost; theres no point in throwing more resources and time into it.
If she has the baby and it is his, they will end up in some sort of relationship. It could be a relationship of love, a relationship of hate or a relationship of muddling along. You might as well let them spend time together and start shaking that out.
Be careful with the idea of pushing criminal charges against her. The outcomes that I see include: mother in jail, mother filing retaliatory charges, or nothing. Of these, the best outcome is nothing.
Next, it is time for you to strategize.
Write down the possible outcomes from this in order from most to least desired. Now cross off the outcomes that are self-contradictory or otherwise impossible.
Now get out several sheets of paper. With one outcome on each sheet, write down the steps that must take place for your five most favored outcomes to occur. Also, write down the steps that must take place for your three least favored outcomes to occur.
Throw away any outcomes that require steps that are impossible.
Once you have your top outcomes to achieve and avoid, as well as the steps that they contain, go and talk to a lawyer. Since you know what you want, you can ask the lawyer to help you achieve it.
Good luck. You need it. Pray to the goddess Anoia and do your best with the knowledge that it may not be enough, and thats ok.