I am sorry to hear about your son.
This is a serious offense. You can read the statute here
. Exactly how serious it will be will depend on the reasons for the accident -- if he was driving recklessly, negligently or under the influence of a controlled substance the penalties can be different.
If he was merely negligent, then this would be a crime of the third degree in Jersey and would not absolutely require a jail sentence The maximum possible penalty for a crime of the third degree in NJ is 5 years, and a $15,000 fine.
If he was reckless, this would be a crime of the 2nd degree. The penalties for this offense would range from 5 to 10 years of prison. If he were under the influence there would be an indenterminate sentence where the top range would be set at the maximum and the bottom set somewhere between 1/3 or 1/2 the max.
In short, he is going to need a good lawyer here, because depending upon the facts of this case, he may not get much of an offer from the prosecutor, and it may have to be tried.