Rachael, If I were in your situation I would handle it this way, but keep in mind, I've learned a lot along the way, so what I am about to tell you is my own personal opinion and you do what you think is right in your situation.
If I were in a situation such as yours, I would meet w/the lawyer and file. I wouldn't tell him that I was filing. I would allow the filing to come as a shock to him. By telling you are filing, you are giving him ample time to prepare himself and his wallet for whatever may come.
As for his mother, I would sit on that for a bit. You want to have your ducks in a row first. His mother isn't going to be happy w/the situation and blood is always thicker than water, so do not be surprised at how she reacts. I'm sure she's heard some rumblings from him already about the situation. Sometimes in situations such as this, you are better off not saying anything to his family about it. They need to actually see for their own eyes what he's been up to and trust me, it will all come out in the end.
Just my two cents.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
