Ashley,
You have not posted, except for your original, guest post, so I am not sure if you will actually see this.
But since this is about a month later than your original post, I need to ask "Have his behaviors changed in any manner?" There can be many different reasons for his "sudden changes in behavior" and I am not going to hazard a guess over the Internet. They could range from some sort of epiphany to medical/psychiatric issues.
Generally speaking, it is unusual for Christians to change behaviors radically, but since the book of Acts details the epiphanies of Saul/Paul, and later of Peter, telling him it was OK to eat non-Kosher meats, we cannot discount them entirely. so you have to ask, and watch him to be sure.
That brings up the second part, the medical/psychiatric part of your question, and here I can be more specific. There are no laws in Western countries dictating how a person should believe; for that we all can be grateful. But in all of these same countries, there are laws about harming yourself, or others. It is a matter of public safety, and of common sense.
I do not presume you to be an expert in mental health, so you cannot make any evaluation. However, you can (and should IMHO) ask the boyfriend if this "God" or "voice" he hears has ever told him to do harm to either himself, or to others. If you do get an affirmative reply, DO NOTHING ELSE except saying a quiet, non committal "Thank you for that".
Then when you get into a safe place, call the local mental health people. They are trained to evaluate the situation, your words and your boy friend, so let them. If you are silent, and he goes off the deep end later, you will blame yourself if he acts out inappropriately.
If he gets "mad" at you for calling in the mental health people, you can always tell him the truth: Because you care for him, you wanted to make sure that qualified people did an evaluation.
Hope that this helps,and that it seems like a common sense approach to your question.
Shalom
