If what you need is to be able to pop in and out of cabs easily, then you want to look for a carseat that can be used without the base. There aren't any bucket seats that you can use without the base that can be installed with LATCH, so you are going to have to weave the seatbelt through no matter what.
I suggest a graco sungride 35 with a stroller adapter like this: http://www.target.com/p/graco-snugrider-infant-car-seat-frame/-/A-12254083?ref=tgt_adv_XSG10001&AFID=Google_PLA_df&LNM=|12254083&CPNG=baby&ci_sku=12254083&ci_gpa=pla&ci_kw=
This little carseat stroller is super light and you will be able to throw it in a cab easliy, instead of having to heave a huge stroller into the back of a cab.
Once your kiddo is a little bigger, how are you planning to get in and out of cabs? Most buckets only accommodate up to 30 lbs, but this one goes up to 35 and 32 inches tall. That's why I suggested it. IMO, I would use this combo for when you are going places in cars/cabs. But, I assume that you go lots of places on foot/ public transportation? If so, I suggest you also get a walking/jogging stroller. This would be able to accomodate your take along items, be comfortable and easy to push (you'd be surprised at what a pia some strollers are to push). Here is my suggestion for that one:
If you got all of those, you'd be looking at about $500 USD. They are all available at stores like Target and Babies R Us which are in pretty much every suburb in America.
There are tons of amazing stroller/careat combo options, but most of them would require ordering if from somewhere, which Im assuming he doesnt want to do.
Keep in mind that when you are looking at carseats, typically the number following the type is the weight limit. For example Graco Snugride 22, only holds a 22 lb baby (which some kids outgrow by 8 months) a Graco Sungride 35 holds a 35 lb baby, pending that he/she is pretty short.