If he were used to using a harness & leash you could stop & exercise him along the way in that. Otherwise, 9 hours IS an awfully long time, but I don't suppose you could let him loose in the car or you'd be a distracted driver.
I really hope you can get an xpen & use it for him at your mom's. You could say it was your next birthday present if the approximately $40 cost is a problem. I got mine for about that, in Nov.
My traveling is only about 3-4 hours door-to-door, but part of that is by train, so weight is an issue, as I have to carry it & a bag of my own stuff onto the train & to my seat. I finally came up with a solution that I thought MAYBE you could adapt to make a small x-pen if he's not a climber. I got 6 cake cooling racks at a dollar store & used ties to fasten them into a cage, with one side left open & later fastened with clips when Honey's inside. The racks were rectangular, so I assembled them with 2 opposite sides sticking up, which can be used as handles. Of course I lined the bottom with cardboard [several layers] & hay and covered the cage with some burlap for privacy. My rabbit is small & isn't a climber, so I could see buying lots of the racks & making a pen out of them for her, but that may not work in your situation.
