Your main weather concerns are heat. Recently when my girl Luna had to go in for a visit and an Op, two of the drives had to be made in extreme heat 42 degrees Celcius. I had no way of avoiding the time of day so I had to plan accordingly. Early in the day before taking her in I made sure to park the car in a spot that would be shaded. Once I had her in the carrier, I went out to the car without her and started the car and ran the air con on full blast for 5-10 minutes to cool the car down. If you’ve got a second person who can stay with the running car it helps. But I turned the car off and went back inside to collect her, added a frozen water bottle to the carrier and brought her to the car. In the car I had her in the front passenger foot well and directed all the air con to the foot well for the drive. I also hung a blanket from the glove compartment across to the back of the passenger seat so that she was shaded the entire time.
It actually was a benefit to me having the frozen water bottle in with her, because the medicine that I got is supposed to be kept cold and getting it home on an hour drive on a hot day I was initially worried, but I tucked it behind the frozen bottle in the carrier with her where she couldn’t get to it and it was still nice and cold when I got home
Otherwise, I put a fleece blanket in the bottom of the carrier, a small handful of hay and that’s about it. If it’s cold weather, you won’t want heat blasting on him in the car. If you’re worried about hydration you can take some leafy greens with, but mine never eat on vet trips.