Any convertible carseat sold today will hold him rear-facing to 30, 33 or 35 pounds, as long as he doesn't get too tall first. It sounds like what you bought was a combination seat--these kind are forward-facing only, harness from 20 or 22 pounds up to 40 or 65 pounds (there is only one that harnesses above 40 pounds) and then convert to a belt positioning booster. Can you return it and get your money back to put towards something else? Saving it isn't recommended because carseats are only good for a limited time, starting at the date of manufacture. If you hold onto it you'll be losing valuable time to use the seat.
Deciding which convertible seat to choose depends on a few things:
How old is Ike right now?
How tall is he and how much does he weigh?
What seat is he currently in, and does he have at least an inch of hard seat shell above his head?
What vehicle (year, make and model) do you drive?
Does Ike ride in anyone else's vehicle? If so, how often?
What is your budget?