I think this is a duplicate question. It could be a three year minimum mandatory depending on certain facts, regardless of it goes to trial. People are not supposed to be punished for going to trial, but they do everyday.
Eye witness testimony can and often is enough for a conviction on this sort of a charge. Your son needs to hire an attorney to fight these charges as there can be a minimum mandatory 3 year prison sentence as part of the punishment.
This response is for general purposes only and does not establish an attorney-client relationship. You should contact an attorney to fully discuss your issues.
Let me add that a person can be charged with aggravated assault with a deadly weapon when there is no allegation of a gun being used. The allegation could be that the defendant used a knife, baseball bat, or other weapon. In such an event, there is no 3 year minimum mandatory for the charge.
If the allegation is that it was a gun, then it is not necessary that a gun is ever recovered. Testimony of a single witness is sufficient to allow a jury to decide whether or not his guilt has been proven. It is impossible to say the possibility of him getting prison time or being found guilty.
This is not to be considered legal advice nor does an attorney-client relationship exist.