J.L.
I don't think you should say anything right now. I would stay with the "dad is in Florida" and keep it light. When and IF he tells you he's moved to Florida then tell your son, but I wouldn't necessarily do a big sit down we have to discuss something very serious - I would over snack say that you know dad has been visiting and now it looks like he's staying down there. He's always going to be dad and we'll figure out when you get to see him. It sounds like dad could be back and forth at any point in time. Don't tell your son his dad is coming to visit until he's on the doorstep. Even though it won't be your "fault" if his dad doesn't show up, you'll be the one trying to mend the heart break.
I know your pain. You want the relationship for your son with his dad, but its nothing you have any control over. All you can do is be the best parent for your son. In that, you will also get the tears and the disappointment in missing his father. You will get the anger and frustration that comes from the abandonment. There was a time, I had a lot of anger that I was consoling children crying for their father who moved to another state. I remember thinking he should be the one seeing this pain instead of me wanting desperately to take it away from them. One day, I decided to see it differently. Yes, he was causing their pain but I was the only one they trusted to share it with. They trusted me with their pain and he would never get to see their true hearts again. He would only get the Disneyland idol worship, but I would be the one they share everything else with.