Sounds like a very serious case. The answer to your question is there is know way of knowing at this point how much time he is facing. It depends on the number of the charges and the grading of each charge as well as a prior record. Be sure he has a good PA criminal lawyer representing him in this matter and be sure that this person is familiar with this type of case. You are able to look up his information on line at a website.
The charges are very serious. Without the benefit of a lot of very relevant facts, the gun alone makes this a case that carries a five (5) year mandatory minimum sentence upon conviction. I have no idea from what you've provided what the stage of the prosecution is. If he has counsel, that lawyer will be advising him about the charges and the way(s) forward. It's up to your boyfriend how much information he wants to share with you. If he doesn't have counsel, then that mistake needs to be remedied immediately.
Mr. Jones and Mr. Dorfman have this question covered. The best help that you can offer your boyfriend at this point is to make sure that he has an experienced criminal defense attorney, or if necessary, a public defender. Good luck.