Hi Jiffa your post as really played on my mind. My son has played up while I am trying to drive and I know how stressful it is. I am often worried I will have an accident.
Just a few thoughts. Is the seat comfortable with a little room for him to move? You know what its like yourself at times but we can move from side to side. Is he hot or cold? We had that problem as the air con works really well in the front but you forget the back, also the sun really hits the back seats as you know. Can he see you or out the window? maybe he likes to see what is happening. Is he getting car sick? I do if I travel in the back, not that I know a way around this for the little ones. The other thing was have you seen the activity mats that hang from the back of the seat in front. They have all sorts of things they can play and look at perhaps that may occupy him. A friend of mine even put her Fisher Price Aquarium in the car, attached to the back of the front passenger seat and her kids love it. Perhaps his pants may dig into his belly while on a long trip, have you tried an all in one for the travel and change him at the other end?
Perhaps your sons problem is more that he doesnt like being buckled in. You know how independant they can be at that age. No doubt you have checked the belt isnt to tight. Is he big enough for the bigger seat ie the booster with the back support?That may give him more room and a bit more space to move around. What age do they start to get scared of small spaces? Perhaps he just hates the car enclosure in general?
As the other post suggested the dummy for us is always a must along with his security blanket. No doubt you have tried just about everything but what about waking up with a happy excited voice right from the start saying we are going to blah blah blah today, wont that be fun. Then only nice happy things to get ready then while taking him to the car saying we are going for a drive, how exciting. Put him in his seat with kisses and reasuring words and saying good boy etc. Then talk like that for the trip pointing things out to him, even if he doesnt seem to be looking? I know it sounds silly but I really believe this helped my son get ready for day care in the early days. I was so upset as I didnt want to leave him but I had to. I knew how his emotions reflected my own so if I betrayed a negative feeling then he too would pick up on that. What I am trying to say is if you get in the car with the natural dread feeling he will pick up on it. Maybe he just wants your attention 100%, which we know is impossible but they dont.
Apart from that do you know anyone with a portable DVD player?? If so that may be worth a try with the Wiggles or Hi5 etc, I know you have tried the music but perhaps the visual may be different. We found the best car toys are anything with a noise and the louder the better. Does he like books? I am not sure how much time you have but have you tried when you get to the other end taking him to a park for a play so he knows at the other end good things are waiting? Or failing that do the park thing before you leave so he hopefully will be tired for the trip home and sleep.
I dont know if I have helped or just suggest everything you have already tried. Please let me know how you are going. I can only imagine you must hate getting in the car!!! Good luck