LOL at image of giant carseat!
Can you try getting one of those bouncy chairs that lies flat tomorrow? At least then he'd be flat - I'm with you on the not leaving them in the carseat for any length of time thing. He could fall asleep in it sitting up a bit, then you could lie him down.
Will he settle on his front? I know it's not advised these days, but I do know some health professionals say it's OK if all the other risk factors (smoking, etc) aren't being done iyswim.
Does he have a cot? Can you try propping the end of it on a couple of books and putting him in there?
Just did a quick google and found this:
"What do I do if my baby is still crying?
Sometimes you won't be able to figure out what's wrong. You may have tried everything and the baby won't stop crying. She might have colic, which can last three hours a day, at least three days a week. Colic usually goes away by the time a baby is 4 months old.
Even if your baby doesn't have colic, trying to comfort a crying baby can be difficult. If you've tried everything and nothing works, try these tips:
Swaddle your baby - W rapping your baby snugly in a blanket can help your baby feel warm and secure.
Sound - T ry playing soft music or singing a lullaby.
Motion - Carry, gently rock, or take your baby for a ride in the car (make sure she's in her car seat.)
Rubbing - Rubbing your baby's back or belly may be soothing, especially she's having gas pains.
Suck - Sucking can steady your baby's heart rate, relax his or her stomach, and calm flailing limbs.
The good news is that babies do grow out this phase. By the time your baby is 6 to 8 weeks old, they'll be better able to soothe themselves and much of the crying will stop."
Can you try a dummy? The sucking might help soothe his tummy without filling it with milk (my DS used to want to nurse when he had a stuck burp...cue projectile vomit and a HUGE laundry pile!).
More hugs, hun xxxxxxxxxxxxx