I understand that he has not yet been arrested.
Your question was what could happen. What could happen is that he could still be charged with the section I have told you about. It only requires a very low standard of proof to charge someone with a crime. The prosecutor only needs probable cause, which is simply a reasonable belief that a crime may have been committed. Whether he can be convicted of a crime is a wholly different matter, as that requires proof beyond a reasonable doubt. But is there enough evidence to get him charged if the prosecutor wishes to do so? Yes.
This may have been an accident. Arguably, however, it was reckless for your son to be handling someone else's gun inside an apartment in the first place.
Please understand I'm not saying he will be charged. I'm only saying it is still possible, depending upon what the prosecutor makes of the matter. And it's the answer to your question, which was "what can happen to him," even though it is the worst case scenario.
At the very least, however, even if he avoids any criminal liability, he would be civilly responsible for the damage to his friend's apartment and the apartment next door.