There is an exit in the design room as FortForever mentioned.
It's also possible to exit from the room where guests board the ride cars. The unloading area and exit are directly across the ride track from the boarding area. Guests who can walk (and maybe chickened out) can walk directly thru the ride car and exit.
For guests with wheelchairs or
ECVs, it's a bit more involved. At the far end of the track from where guests enter the room there is a small elevator that goes up one floor to a short hallway over the track. Then, there is another small elevator to go back down on the other side of the track o the exit.
Depending on his transfer abilities and his ability to keep sitting balance on a hard, somewhat slippery seat, it may be possible for him to ride Test Track despite being a paraplegic.
At the hallway area, there is an alternate loading/unloading area at the seatbelt check. The ride car at that point is higher, so boarding there is kind of like going over the side of a bathtub. A wheelchair can be brought right up to the ride car; transfer in and then the CM parks the wheelchair for you.
They have some tools that might be helpful, including a grab bar that can be put onto the car to help pull in. Getting in is not real easy, but gravity does help. Getting out is more difficult since you are working against gravity.
There is a ride car that guests can look at before going into the attraction to help plan whether or not it is feasible to ride. Just ask the CM at the entrance.
If you've never ridden before, going thru the single rider line is usually a faster way for a member of your party to check it out to see whether you think he could ride (check out the seat belt check area while you are there for a few seconds for the check). You can also tell the entrance CM that you need one of your group to check the ride out first for your companion.