Well, he survived the trip! I figured I'd update you all on how it went since you were all so kind and helpful to me. I ended up emptying his tank halfway and pack it with a towel into a Cardboard box, since I was short on plastic bags. If I ever have to travel with him again, I'm definitely investing in lots of bags because this was a new kind of Hell for him I imagine. The whole journey took two days, due to timing and relying on other people for transportation, so he only spent about 3 hours in the vehicle each day. When I got to my parents' house and got him set up, he seemed to be fine. I didn't feed him that night since it was fasting day, but he ate everything I gave him the next day, so that was a relief. However, closer to the end of our stay, he stopped eating, and I got kinda of worried because we were travelling again in just a couple of days and I wanted to make sure he was as stress free as possible going into it. He ended up going for a total of five days without eating, two of them were another two-day journey home. This time, the weather was MUCH colder, (about -20C going up, and -40C coming home) Unfortunately, since we were in a rush to leave I couldn't let the heater cool down slowly before packing him into the car, and on the journey hoome at some point, the glass of the heater cracked. I'm so glad I bought him a new tank and got it set up before trying to plug that heater in, because I'd definitely have one fried fishy right now if I didn't.
As for the new tank, I bought it on sale just after Christmas, because I wanted a filter and a light for him and it ended up being cheaper and more sensible to buy a whole new tank at the time. He's now settled into it and learning to play with the water flow from the filter for fun. I think his fins may have been damaged a little bit during the journey, but it doesn't seem to be anything major. He's eating again and even bit me last night after when I put the jar of pellets away, (he'd eat forever if I let him).
Here's some pictures of the new tank: