I haven’t had FFS myself. But I’d recommend asking about recovery times, surgery prep, medications you aren’t allowed to take leading up to it, after care that must be done once you’ve had the operation, possible complications and what can be done in the event of those complications arising. If you’re traveling for the surgery it may be worth asking how long you should stay in the area after surgery in case any follow up is needed (probably not necessary but it was for my SRS so I think that might be worth asking).

It would be good to go over expectations for results, specifics of what procedures he thinks are worth pursuing. FFS usually entails a combination of procedures.