Three options I can think of off the top of my head:
1: the Hope Car Bed. Allows side lying position up to 35#. He may be too tall though. Takes up at least 2 backseat seating positions.
http://www.eztether.com/index.php/product-info/hope-car-bed
2: The Roosevelt. Forward facing only. Has a neat system of shavable foam inserts which can be custom shaped to the s=child and glued to the shell under the cover. These would allow the torso portion of the shell to be built out to so that his body sits further forward without pushing his head forward as well.
http://www.eztether.com/index.php/product-info/roosevelt
3: Modified EZ on Harness. Back or tummy lying position. This is the most affordable option if paying out of pocket. Takes up at least two backseat seating positions.
http://www.especialneeds.com/e-z-on-modified-M203-mini-push-button-vest.html
*It might be worth contacting Merritt to ask if the customizable foam can be used on the Jefferson rear facing seat as increased head size and abnormal bone development are both reasons to rear face as long as possible.
Now, as a special needs tech, I can tell you that when a child is in a Spica cast, the angle of the cast often causes them to sit in the car seat in a way that pushes chin to chest or chest to cast and makes breathing difficult. When that happens, it is accepted practice to fold a towel up behind the child's shoulders to arch the back just enough to let the chin lift. This isn't best practice, but when dealing with SN situation sometimes there are zero carseats that don't force parents to compromise some crash safety in favor of solving a problem that's much more immediate than a crash that might happen. Since it may take some time to determine if there is a carseat that is just right for him, I wanted you to have at least a temporary solution for his current car seat that has at least been approved for some situations, just in case it gets to the point where his breathing is compromised enough that your only other option is to not use a seat at all.