My father sees things that are not there. He sees words & letters on objects. Please help.

He will spend hours scribbling all directions with different color writing utensils on things such as paper, junk mail, toliet paper, etc, he'll say it says things, hidden messages. He's 46, has history of mental illness (PTSD, Depression, Anxiety) & past drug use. Myself, mother & siblings are concerned. When we try to talk to him about it he thinks we think he's crazy. About 4 or 5 years ago, my mother cheated on him during a bad time, meth was involved. They got back together & have been together ever since. I know my mother hasn't done anything with another man since, they live with me, she's here everyday. They don't even own a vehicle & my father claims she has some boyfriend, that she's still cheating someone she is talking to. He won't drop it or move past it. We don't know what to do. We are concerned for him & our safety. He even takes people's memory cards from phones, computers, etc & GETS all information from it & saves it to his stuff. Like to hold it against us if something ever goes wrong. We need answers. He also will pick up rocks in the yard, he'll spend hours picking up & going through rocks & will say they have letters on them. He has probably 10 plus coffee canisters of just rocks. Please help.