Hi,
You don't have to decide when you file, but you WILL have to decide before it can be finalized because if you want to change your name you need "name restoration rights" in your divorce decree. (Gone are the days that you can just walk into the Social Security Office and change your name at will).
I'm in a similar situation (but a year and a half into the process... ugh) and have decided to hyphenate my maiden and married names (probably what I should have done when I got married in the first place). Socially, I'll probably go back to using just my maiden name, but having my son's last name still in there will keep things less complicated for school registration, insurance etc.
Another option, but it would require a lot of cooperation on your ex's part (not an option for me) would be to petition to have your SON's last name changed to a hyphenated version of yours and his dad's last names, then change your name back to your maiden name. That kind of name arrangement is very common for children who's parents were never married and is standard in many Latin cultures (mom's maiden name as middle name).
Good luck with this,
T.