The simplest thing would be to just keep his bag on a diagonal until you get home. (What's the temperature there though? I'd be worried if it's very cold outside.)
If you're concerned though.....
Are you still at the petstore? If so, go back in, and ask for more conditioned water. Or, if you're near a different petstore or a Walmart, you can get a bottle of "Betta water." It's just treated water, and it's fairly expensive, but at least it's been treated. If you can get this, add a BIT to the bag, so there is more water volume.
I would NOT do a 100% water change, as the sudden change in water chemistry could cause shock. (This is why we acclimate them when doing even just a regular water change.) But you can add a SMALL amount of water, just to increase the total volume.
But I did a rescue once, and was about 2 hours from home. I kept him in the plastic cup that he came in, and wrapped a coat around the cup to keep the temperature stable. Once I got him home, I set up a tank, and started the regular acclimation process....