Ex wants to move son

My ex wife wants to move back to FL after living here for 3-4 years with my son. We finally divorced officially last year. She says she’s not happy here and wants to take my son to florida where her family is.
My son is with me half the time. She lives close so it was easy for us to set up a fair schedule. There is no court order for custody or any order for visitation. We had an absolute divorce cause we had no visitation or support issues.
How do I keep my son here? Can she just pick up and go if we have no court order for custody?

You can file an action for custody. If she attempts to leave you will need to file an emergency motion to prevent her from leaving with the child.

And what do I do if I can’t afford a lawyer?

You can file pro se (without a lawyer) for around $100-150. When I filed for emergency custody (in 2009), it cost me $117, including 2 notary fees, sheriff’s service, and the filing fees. You would need to get the papers from the self-help center at your courthouse (if they have one), or possibly online, fill them out, have them notarized, make 3 copies, go to the courthouse, file them, and have her served. The emergency custody is what you would want to file to keep them in the state. The ‘regular’ custody papers is what will eventually establish the permanent custody.

You may file on your own, and may want to consider using our DIY service to assist you.

If the ex is planning to move my son out of state this summer, when should I file to keep her from leaving?
Would me filing for modification of custody keep her from moving?

I suggest you file a motion to modify now and cite her intentions to move as the substantial change in circumstances.