At that size, I don't know if your child will fit in any higher-weight harnessing seats. If he fits in the Frontier it might be a good option if you can get it installed tightly; it does not require a tether until 65 lbs. The Safeguard Child Seat also does not require a tether and harnesses to 65 lbs. and the harness is a smidge higher, but it isn't as roomy for bigger kids.
If you can find a suitable spot to fit a tether anchor, that opens your options up a lot, since there are two vests you can use but both require a tether in this situation.
If you have a lap/shoulder belt in the front, a booster seat can go there (move the seat back if you can.) If you have a lap belt only there, you can use a harness or vest there and tether temporarily to the seatbelt behind it (again, with the seat moved back if possible.)
The vest and harness I'm talking about are the Ride Safer Travel Vest and the EZ-On 86-Y Harness. Either one can be used with a lap belt and tether (either retrofitting a tether if you can find a spot, or using the front seat and tethering to the seatbelt in back if that is the only way to tether.)