Hi C.,
It sounds like you are already off to a good start- you seem to recognize that it will be hard for you to deal with this situation but you are trying to not put your issues with your ex on her, which will make it hard for everyone, including your son.
Keeping things casual and not confrontational is a huge way to start. Meeting for coffee or in another informal setting is great.
You already got some great advice. My biggest thing to add is this- ask questions. This keeps you from making assumptions or just 'telling' her things, and makes you seem interested in her perspective.
Asking her "what are you hoping your relationship with my son is like", "how do you handle discipline with him?", "do you have any issues you'd like to bring up with me", "is there anything I can do to help make your time with him easier". Things like that. Shows that you are accepting her presence in his life, shows that you are willing to work together, and shows that you realize this is a two way street. If she is bad with your son and combative with you, you know you'll have bigger problems. But the best case scenario is that this sets the stage and will hopefully keep things pleasant or at least civil.
I think most girlfriends in this situation already feel at a disadvantage with the mother of their boyfriend's son, and might start off pretty defensive. So anything you can do to disarm that can only help you in the long run.
I hope it goes well for you (and your son!).