This morning he had breakfast in the car and then when I got him out he did a poo straight away. Will try again tonight. He needs to go in the car for an hour tmr so I might give him breakfast in there again get him out try to get him to poo, then head off.
Think it'll have to be a mix of slowly getting him used to it, and just making him travel in the car.
He's still very young. Is it possible he's frightened by the noise/movement of the car as he's pooing as soon as you start moving?
In your situation I'd start right back at the beginning by getting him to go in and out of the car while it's stationary and the engine is off. Let him play, let him go in and out of the crate with lots of treating and positive reinforcement. Then when he's happy with that start the engine but don't move the car, treating when the engine is turned on and just building up his confidence until you feel he'll be ok with the car moving. Keep the journeys short until he's happy and just keep rewarding him.
I could be wrong and someone else might give you a quick fix for this, but if he's pooing every time you go in the car I would think you need to give him more time and he needs training to accept the car rather than just keep repeating the same behaviour.