Here is the law for Involuntary Manslaughter in Missouri and while your son was charged with attempted it would seem a bit excessive unless he had alcohol in his system as no one died due to his accidental blow out which should not have been held against him in my opinion.
Involuntary manslaughter, penalty.
565.024. 1. A person commits the crime of involuntary manslaughter in the
first degree if he or she:
(1) Recklessly causes the death of another person; or
(2) While in an intoxicated condition operates a motor vehicle or vessel in
this state and, when so operating, acts with criminal negligence to cause the
death of any person; or
(3) While in an intoxicated condition operates a motor vehicle or vessel in
this state, and, when so operating, acts with criminal negligence to:
(a) Cause the death of any person not a passenger in the vehicle or vessel
operated by the defendant, including the death of an individual that results
from the defendant's vehicle leaving a highway, as defined by section 301.010,
or the highway's right-of-way; or vessel leaving the water
I hope that this information is helpful.