a 19 inch torso seems really long! I would think he would be about to outgrow his current seat with those stats, as well as the myride and true fit (depending on how you measured him). Our Apex 65, I believe, only has 19 inches from the bottom of the seat to the highest harness slot - which is pretty close to as tall as they made them when we bought it. And it was advertised to be for kids who outgrew their 40 lb. car seats - though I have a hard time imagining a short enough kid who also weighed 64 lbs still fitting in it.
ETA: phew! I was thinking, dang, she has a huge 16 month old!
If I were you, even know that we know he isn't that tall, I would get something that will keep him rear-facing the longest, and then FF later on the longest. That same seat can be used for the baby in the beginning and end of his/her car seat time, as suggested, and use the Alpha Omega for the middle period while your older DS is using the new seat.