Giving wet greens during breaks should be enough, especially if he is good about drinking once you get to your destination.

How long is your total travel time?

I just moved cross country with my four bunnies. It was 6 days of driving, with 8-12 hours of driving each day. None of them would eat or drink while the car was moving, but halfway through each day we would take a longer break and offer wet greens. I never syringe fed during the day, but did have to syringe and give fluids to one bunny in the morning and evening for most of the trip. Everyone else was fine though. Basically I tried to get something in their system every 6 hours (max, preferred every 4), even though it wasn’t always much.

So I think if you can offer him wet salad before and after the flight, and possibly during if he will eat on the plain, he will be fine. You can syringe either fluids or diluted critical care. I think a couple mLs would prob be all you would want to deal with on a plane. I just think syringe feeding on a plane would be a nightmare (my understanding is that most airlines aren’t allowing rabbits as emotional support animals anymore, so they need to stay in their carrier, I could be wrong though).

Since it’s only 1 day of traveling I think it’s less of a concern, because when you get there you can give him whatever he needs. I talked to my vet before we traveled and we made a plan for everything, so I recommend chatting with your vet too. She set me up with sub-Q fluids and pain meds in case any one got too dehydrated or stressed.