I would just walk over by them a lot, and once hes used to them at a walk, go to a trot, etc.. Mostly just keep his mind off of it, make him walk/Trot/canter faster, slower, stop, back up, etc while going towards the window. This helps me when my horse spooks. Good luck!
Maybe put some mirrors near his paddock (like in an adjacent paddock) and put some in your arena, he might be spooking at his reflection or the light? Otherwise Park your car near where you're working him and ride around your car (or float) so he get some used to the reflection of lift off windows?