I put a mirror up on the backseat so that my son could see himself and be entertained that way. It was also great because it had music and toys on it, and it was nice because while driving I could see him and he could see me. Made things so much easier.
Also your 5 year old is going to be riding next to the baby so maybe they could keep the baby happy for awhile.
Bring lots of toys and things that will keep baby happy, things that work great at home, bring them.
I am sure the baby will be sleeping a lot of the way because its such a long drive.
Also maybe the carseat needs more padding? And maybe baby is too hot, too cold? He is probably just mad because he wants to be with you and not strapped in lol. My son always hated that.
Have fun, and hopefully he will surprise you this time and be perfectly happy!