My son was born 11 years ago before my exwife and I got married. He got her last name. We got divorced a year ago and I got custody of him. I want to change his last name to mine. I have talked to him about it and he is OK with having it changed to my last name. She is about to get married again and I was wondering do I have to have my ex wife's permission to change his last name to mine or is it something I can just go down to the court house in Marion County Florida and do?

Thanks

Yes, you will need her consent to get it changed. You could try to get it changed without her consent, but it's a difficult and often expensive and ultimately fruitless process.