Henry went thru a time of being the same. We removed the crate. He now travels in the boot.
To train him to get in the boot, we put a hot dog sausage in the boot, on the drive, without him seeing us put it in. We then opened the boot, he found the sausage, he then jumped back out. We repeated it several times, never letting him see us putting the sausage in, and over several days, before we went on a journey. He now jumps in ok. The only problem we have now is that he thinks every car boot has a sausage in, and he's jumping into anyone's!
It is a very frustrating problem. I would spend half an hour at a time trying to get him in the boot before training. I think the key was to get him hunting for the sausage, rather than rewarding him for getting in.
Good luck.