With all honesty the LS400 can be a daunting car for a new driver to handle, especially the older models. RWD with a good bit of power and a soft suspension that can be a little hard to handle in emergency maneuvers.
When I was a kid a friend of mine got a new BMW 3 series coupe for his 16th birthday. Certainly a more stable handler than an LS400 and he managed to flip it on straight ground. Don't ever think any car is teenager proof.
Everybody makes mistakes though, I feel for your son...
Cars can be replaced, be glad he's okay.
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i second that be glad that hes ok i still underestimate the power of rwd from time to time