The first thing that your son MUST do is remember that anything he says to anyone may be used against him. That includes statements made to even his parents. The second thing that you must do is to have him consult with an attorney, whether private counsel or public defender. The Dauphin County Public Defenders' office is staffed with may able bodies and eager attorneys. But the private sector may be more accessible and able to offer more services. Aggravated assault with a gun carries a mandatory minimum of 5 years, which may be waived by the prosecutor. This is very very serious. Do not allow him to give any statements to ANYONE, in particular law enforcement without an attorney being present.
To answer your first question Agg Assault is a serious crime and the sentencing guidelines to permit a jail sentence even for a first time conviction. As for the second question, yes a public defender will help him if he is eligible. PDs are darn good attorneys who have a heck of a lot of experience in the trenches, which leads me to whether you should hire a private attorney. Whether you take out a loan or not is a decision only you can make. While Public Defenders are good attorneys they are often over worked and their case load is very large, as a result they often don't have the luxury of being able to spend a lot of time with clients as does a private attorney. If one can afford quality private counsel I think its a good idea. That said if they can't, the public defender's office has highly qualified attorneys working in their office. There are some good attorneys on AVVO out in central PA. F. Dean Morgan in the Hershey Area comes to mind. I’m sure there are others too. Check the listings here on AVVO and I'm sure you can find someone who can at least give you a few moments of their time to discuss the matter.