If my husband attempted suicide and was being charged with possession, how can they charge him if the weapon wasn't even his?

This is my husband’s second suicide attempt. He is a war veteran that suffers immensely from depression, anxiety, and alcoholism. 11 years ago, he caught a felony drug charge. Back in December, he found an unloaded weapon and threatened suicide. I called police, and received a Swat team. They took him to our local mental health facility, where he was held for 48 hours and released. Late December, he was arrested on a warrant for having possession of the firearm by a convicted felon and here we are. I'm confused about how they can charge him for this when a) the weapon wasn't even his nor on him, but in the house as display, and b) we have absolutely no ammunition in our home, as I removed all of it after his first suicide attempt. He has been clean and sober for 4 months now, and regularly goes to group therapy, a psychiatrist, and therapist (on his own will).