The actual Sit And Stand brand ones have a bar that goes across the seat to clip the car seat to and a strap to put over the foot of the seat. As far as growth of your older son, keep in mind that he will start slowing down soon. DS1 was very similar to your DS at that age, but now at almost 5, he's just barely 40 lbs.
Hi! I'm Shannon
Wife to J
Mama to a couple of crazy boys - L (12/07)
and E (2/11)
and Meatball coming 4/14