Those Cosco seats (like the Apex) say they go up to 65 lbs but the harness starps slots aren't as high as other seats, as you are experiencing.
It definitely sounds like he is not ready for a booster maturity wise, I agree.
There are other seats but like you say, they are pricey. Would anyone be able to lend you one? The Radian seats are the tallest with the highest harness slots/weight limits AFAIK
. Could you contact someone to see if a free/low-cost seat would be available to you through your local CRST/clinic? Or would medical insurance cover a special needs seat? I'm in Canada so I'm not 100% sure on how things work in the USA but I've heard people speak of getting free seats.
I'll look into it more for you tomorrow, coughing kid just woke up so gotta go.