Just thinking here... here's one potential approach:
Approach his best friend(s) and let them know the 411. Approach your (and his) male family members with the same. Then, with at least 4 of them plus yourself, confront him (non-aggressively) when he is most likely to not be in a foul mood or armed, and let him know (in no uncertain terms) that it's time for either a sit down or a drag-out. Let him know that it's time for an intervention, and either he deals with his friends and family peacefully - people who have his best interests in mind - or you will can deal with it outside of the family.
(In yesteryear, it was supposedly not uncommon for such "interventions" to involve one or two people having an extended "lecture and a beating" with such an individual, I've heard.)
Make sure he is clear on your family conditions (temporary removal of firearms from the house, absolutely no drinking, counseling alone and with wife, etc.), as well as the alternative (ie what the law will do to him). Make sure his wife is in complete agreement with you as to your approach: she might prefer doing something else (just having the cops deal iwth it) or she might be operating in "beaten victim mode" (there's another official term for it, but I'm tired and can't recall it) and would prefer to side with him out of fear of losing him. However, I thin that if you approach this in a level-headed manner, with people he trusts or may hold authority in his life (father, brother, etc.) you may be successful.
Is this person in his 20s? A person's 20s is often when mental illness manifests itself. Is there a history of it (dementia, Alzheimer's, bipolar, depression, etc.)? If no, then there's probably something distinct that is causing his freak-outs. He needs to address it, and come to terms. Often in America, men are taught that they've got to hold their problems in. Yes, that's how we tend to deal with things (internally), but it is also the case that many people aren't able to do so. When those people hold their problems in long enough, you are presented with a sad example of psychosis, such as Cho or Witman.
I should note that it is ABSOLUTELY important that he's sober when you approach him (and hopefully alone/not with his wife), and that you and everyone else gets the impression of his honest intent of compliance. He will need to have people - family and friends - checking up on him to make sure he's complying with the agreement.
This is, at least, what I would attempt to do for my friends and male family members. Better to work it out before the law gets involved. It would NOT be a bad idea for him (or his wife) to step outside the scenario for a while (go live with a family member) until his problem is determined and reconciliation is made possible.
Keep a cool head, THR folks: you never know what the problem could be. He could have a brain tumor that is causing his erratic behavior. If that's the case, and it's something "fixable", it would be horrible to deprive him of his liberties over a medical complication. Frankly, I am surprised you all are as ready as you are to have this guy call the cops when there are apparently loving family members close by able to help (that's my read on the situation - obviously I can be ignored if it's not true).
Once a threat is conveyed, it's time for the law. Intervention by male family members may result in a lot of work for the coroner. Events like the Delevan, Wisconsin tradgedy do happen. It's sad, but this guy has attained "perp" status and needs to be dealt with quickly and, with luck, safely by the local pros. 