My daughter was the same way. I'm trying to remember when it stopped and I think she started tollerating trips around 4 or 5 months. My husband's mom lives about 40 minutes away from us, and we go every weekend. So, you can imagine, we couldn't have a crying baby all the time. We would time our trip around her schedule. Right after she ate and then fell asleep, we'd jump in the car and go. That was the only way we could manage such a long trip. For the shorter trips, I would sing, we'd switch up her toys, and then when possible, like you said, someone would sit in the back with her.
She's now 9 months old and can ride in the car pretty well. But, it didn't just go away immediately. We still would have episodes where I would have to stop and we'd just get out and walk around whatever store we happened to be near until she had calmed down.
I know it's tough. I've found that if I sing that would sometimes calm her and she'd go to sleep. I don't think it's unusual for a baby to cry in the car. I'd heard all those things about them loving the car too, and just didn't find it to be true. I think they're back there and alone and don't know where you are. Recently, on a 2 hour trip, I gave her a shirt of mine and that seemed to help. They just want to be held and can't understand why we don't pick them up.
Anyway, those are the tricks I can think of. As for him being colicy (or high-maintenance), have you tried Happiest Baby on the Block? That calmed our daughter right down and I know of another family who had a colicy baby and said when they discovered Happiest Baby on the Block there was a drastic change. They sell a book and a DVD, or you can Google it and read about it. Basicly, you swaddle them, shush loudly in their ear (really loud, so they can hear you over their crying), put them on their side, sway them, and finally give them something to suck. You're supposed to do it in that order. By the time we got to the shushing our daughter had usually calmed down.
I don't know of any other websites, but I check Dr. Sear's website a lot for info when I have questions.
I hope that helps!