Oh, sounds a lot like my DS (22 months today) at that age, although thankfully he was an early sitter and started getting happier when he could sit unassisted. He got happier still when he could crawl (6 months) and MUCH happier when he started walking (8.5 months). You've been given a lot of really great advice here... the other thing that was a godsend for us was going to places with older babies/toddlers, like the local indoor play centre and story time at the library. He just drank in the sight of the 'big kids', really seemed to be studying them for cues on things he could try to do. (Meanwhile he had zero interest in other babies his own age).
And just an aside, on the carseat thing - some babies hate the convertible carseats (the kind that go rearfacing now and forward facing when they're older) less than they hate the infant seat, so if your little one is still in the "baby bucket", you might want to look into a convertible seat earlier rather than later. They sit a little more upright and less cramped (and then once they're a little older, 7-8 months or so, it can be reinstalled a little *more* upright to really let them see better). My DS did better in the car once we switched, although in his case we HAD to since he was outgrowing the bucket by 5-ish months.
