Have you started small and increased the height incrementally? If not, go back and start over. If he knows the crate/kennel command, start with that, with his crate on the ground. Does he know the spot/place command? If so, use that command with a stable platform that you can slowly raise.
Some other things to consider; Is your truck particularly high? Is he overweight? Is he physically able to make the jump? Do you ever hunt him off a platform?
If all else fails, consider getting a ramp. I used one with Jack when he was younger. If you go this route, as with anything else, start small. Lay the ramp on the ground and let him get accustomed to it. Encourage him onto the ramp with hot dog slices if necessary. Then raise one end a few inches and repeat. Do this slowly and methodically.
Whatever you use for training, be it platform, ramp, etc., make sure it is stable. Nothing turns a dog off quicker when learning to jump up onto an elevated surface than a shaky setup. Other than that, go slow, and use hot dog slices as a reward. Good luck.