If you are on Facebook, "Car Seats for the Littles" is a great resource:
There is actually an album of photos called, "Car Seats in Ford Mustangs" that includes pictures of rear-facing car seats installed in Ford Mustangs. I would investigate a Diono Radian (which one depends on the size of your child: the R100 rear-faces to 40 pounds, the R120 to 45, the RXT to 45 and has headwings) and get the optional angle adjuster (it goes under the back of the carseat and makes it sit up straighter). Several people have reported no issues with having Diono Radians in the back of their Mustangs. There are a couple other options that *may* work, but none that will work as long.
I don't like to get all gory, but what can go wrong forward facing is pretty drastic. Because of a combination of their heads being so large compared to their bodies AND the fact that their spinal column is not done developing, children who are forward-faced earlier than they should are at risk of severe injury. Sometime between 3-6 years of age this no longer becomes as big of a concern, but children your son's age are up to 532% safer rear-facing than forward facing. In an accident that age of child can actually become internally decapitated which can result in permanent paralyzation or death. There are definitely options to keep him safe!