Yep we're dealing with that too. So far my solution is to avoid driving anywhere. I do have to get groceries now and again though and have just had to deal with it. It's so stressful, even for a 10 minute drive.
My other possible solutions are:
Set him in the car seat in the house for short periods so he gets used to being in it
Go for short rides to get him used to being in the car
Change car seats (I also have a convertible & another bucket I can try)
I've tried shushing, loud music and a soother, he won't take it, nothing helps. My ds hated his car seat til he was 3months and dd hated hers for a year so i don't know if anything will work. I have heard of people getting chiropractic help for it so that's maybe another option.
I would like to go to my SIL baby shower in 2 wks and it's a 1.5hr drive and for Easter we want to visit our extended family 2 hrs away so I'm hoping something gives before then!