You need to get out of your head the idea of being "fair" to your current firm. What you have with them is a professional relationship based on free mutual exchange: your labour for their money. You do not need to be "fair" to them, you need to be "professional" with them. Which could mean staying or going, but whichever you do you are doing it for your own benefit just as they are employing you or not for their benefit.
You may have a personal as well as professional link with your boss, but what you have in this situation is mainly a professional link - after all, he has made his decisions and plans for his move before telling you: that means that it is mainly a professional link, rather than a mainly personal link where he would have talked the plans over with you before making them. If your boss has any sense, he will have a back-up plan for you saying "no" to moving. That means that he will also be able to have a back-up plan for you saying "yes" but moving on at some point in the future. (It also sounds as though your boss is the rainmaker, so he has a lot of power in this situation, and which also means that I don't quite understand what you mean by "not wanting to burn the bridge with him". What bridge, and why do you not want to burn it, given your plans for FIRE?)
All of which means that you are free to do exactly what you suits you best. As far as I can see, each of your three options displays a deep reluctance to make the move to working with your boss, and FIRE seems to have very little to do with that reluctance. That suggests to me that you really would prefer to stay where you are. They question then is: will your firm want to try to fight to retain as much of the work in your area as possible, or want to wind it down? If the first, they should be offering you boss-level money to try to hold on to existing clients and bring in new ones, and might also be looking to bring in a new lawyer who can bring a following with them. If the latter, you will be out of the door as soon as you are no longer earning your keep and the loose ends of remaining clients have been tidied up. Whichever it is, your firm will be able to look out for themselves, so don't worry about them.
You asked specifically about what to tell your boss. It seems to me that you need to tell your boss that you will not be going with him and that once he has made a formal announcement you will need to discuss your position with the firm and obviously aren't in a position to talk to him about that. There's no reason why you shouldn't indicate to your boss that you and your wife have been looking at making some major life changes in about a year and that those plans have factored into your decision not to go with him.
The other side of the coin is that you also need to work out what to tell your firm once your boss has announced his plans. I'd start out by listening to them, finding out what their plans are for dealing with your boss's departure and then responding to them according to whatever you think your own best interests are.
Good luck, and I hope you come back here to tell us how it all goes - I for one am fascinated.