Does he cry pretty much every time he's in the car? I ask because we found DD2 to really, really hate the infant (bucket) seat AND the Evenflo Triumph in DH's car, but NOT in mine. The only thing we could figure was that she was in DH's car primarily at night. We figured it was car headlights from behind, or that she'd had a long day and was fussy, it was dark and she couldn't see her sister, or well, we came up with lots of reasons she might be unhappy.
Turns out, though, she just hated her seat. Now that she's in a different RFing convertible (Britax Roundabout, not that it matters really), she's much, much happier.
I have no idea if the angle was different, if the seat was situated so that the headlights of cars behind DH's car were shining in her eyes, or what.
But for us, changing the seat made all the difference.