I can appreciate and understand your concern over your son's safety.
The low torsional rigidity is necessary to promote equal rear tire loading. The front end is, in effect, decoupled from the rest of the car, allowing a large part of the reaction torque from the engine/trans mounts to feed back to the rear and partly cancel the tendency to unload the right rear. I'm afraid the only way to greatly alleviate your fears would be to put your son in a suspended car. This would allow a cancelation of the driveshaft torque...by suspension asymmetry...without the structural problems which concern you.
My own son has been running a tube frame sedan, but is building a short wheelbase altered this winter. Without the low torsional rigidity of the long wheelbase dragster, the short wheelbase altered is usually considered quite squirrely (to put it mildly). I plan on working with him on an asymmetric suspension, however, which should make this a relatively safe ride.
I devote quite a bit of space, in my blog, to suspension asymmetry:
http://home.earthlink.net/~whshope