C.V.
Well. It's really not quite that simple.
As it stands, the LATCH system has a weight limit and even though there are seats sold which anchor in that SAY they have weight limits up to 110lbs, they fail to describe that the LATCH weight limit is 65lbs on the lower anchors (child and seat combined). Once the weight limit is reached, you are supposed to detach the anchors and use a seatbelt with the booster. To do otherwise can cause injury to the child. In a crash, the booster moves with the child, cradling him and preventing the seatbelt from taking all of his weight.
Here's some info about LATCH. http://saferide4kids.com/latch-weight-limits/
Here's a good seat that would work. http://www.amazon.com/Evenflo-Maestro-Booster-Seat-Wesley... I'd keep him in a 5-point harness as long as you can (he can gain another 15lbs or so), and then remove the harness assembly, detach the anchors and be sure he's properly restrained using a lap/shoulder belt instead of the 5-point harness.
Good luck!