I'd take him on a few short rides that end in a positive way 1st.
I would put fruit in there instead of water for the car ride
Make sure he isn't by an airbag and if you can, find a way to tie his cage to the seat so that if you stop, it doesn't go flying.
I keep my travel cage's perch low so that if there is a sudden stop, my bird doesn't have far to fall.
Some birds like to see out the windows, others do not. Mine flaps wildly if she can and that is dangerous in a travel cage due to her size. In a smaller car with lower seats, I can just leave the windows uncovered, but in the van, the seats are higher and windows bigger, so she can see out. Consequently, I tape the windows with paper (just the ones to her right and left).
Have you talked to your parents about not using Teflon/PTFE/PFOA/PFCs when you are there? That's the hardest part about long-term visits--- most people use chemicals in their homes that can easily kills your bird (teflon/pfc/ptfe/pfoas being the most dangerous) but also standard cleaning products, candles, Glade plug-ins, air fresheners, oil warmers etc. Glade plug-ins are super unhealthy for them.
You should talk to them about these dangers now, and offer to bring alternatives if all of their pans are non-stick and if all of their cleaners are not bird-safe. Do they have any pets? If so, you might also consider baby gates etc (although that will be of little use if he glides over them lol.
I have a cage that I leave at my parents' house for long visits and it is the same as one of her cages at home. You can get pretty cheap flight cages. My uncle leaves one at my parents' house for his jardines as well, but his is smaller and easier to take apart.
Another weird thing we have to do when I visit my parents is block off a door-frame at night. The kitchen and dining room have an open door frame that used to have a swinging door but no longer does. In order for Noodles to sleep at night with all of the chatter/commotion of company, we use an old door (off the hinges) to make the room darker and dull some of the noise from the kitchen. We slide it out of the way in the daytime, but put it up if we will be gone or if it is getting loud and it is her bedtime..Without it, she was wanting to join the party at all hours (despite already being covered).