My elderly neighbor is showing signs he might not be mentally fit to care for himself. What should I do?

More than likely, he was on his porch hollering for people that weren't there because he was drunk.

You should bring him a meat loaf or ust a plate of home cooked food and maybe he will be inclined to talk to you.

Then ask him if he needs help.

Maybe you can call a local church to see if they could visit.

I would call the police.
You could do this one of 2 ways.
Call the non emergency number and describe the situation (you do not need to mention drug use) and say you are concerned about his welfare and safety.
or
During an episode as you described call the emergency (911) and again describe the situation and they will dispatch officers. They would then most likely take him to the hospital for evaluation.
This would probably get a faster response than calling Adult Protective Services at this time.
If your area/county has a Mental Health Department you could call them as well and discuss options, they might also be faster than APS and not as frightening as the police.

Call APS and maybe the police as well, since APS probably won't be out there immediately, and if he's doing anything that is a danger to himself or others, or making threats to people you MUST call the cops as this is the right thing to do and it may get him into a hospital and he may get care faster. Please move on from the snitch mentality, this man has bigger issues. You can be there to explain that you think he may have age-related cognitive problems. Any doctor who treats him will need to know what he is putting into his body so that they can anticipate drug interactions with any other medications he may get prescribed. FYI your body reacts to weed like any other chemical -- because that's what it is. And weed isn't regulated so one has NO IDEA what is actually in the drug he is smoking. Or even if that's the only thing he is taking.

Kudos to you for being a caring person! If you are able and feel safe doing so, see if you can talk to him in a lucid moment to find out if he does in fact have any relatives or close friends who may be appropriate to advocate for him. Get their contact info and call them. If he has no one, then social services will eventually designate a legal guardian and they will act in his best interests. Bless you!

Unfortunately, the police are often the first step to getting someone help. If he's actively hallucinating, they will take him to the ER and the hospital will be charged with making sure he is discharged into a safe situation. You can also call Adult Protective Services and tell them the situation and ask to have him checked on. I have not found that avenue to be very useful but it's something you can do.

The way to get care for people who can't care for themselves seems to be 1)you wait until there is a crisis, 2) get the person into "the system" and 3)hope they don't talk themselves out of a safe discharge.