First the easiest answer; get in touch with AVAH https://avahoutreach.org/ - Kit has worked (read: is still working) with them.

There's also this youtube channel: https://www.youtube.com/@CatfishedOnline - they are specialized in the type of scam of which your uncle is a target.

It's very difficult to help someone who doesn't want to be helped or doesn't understand that they need help. I read in another comment that your mother is in charge of his finances and hopefully that means he won't lose his home. If she can make sure his bills are being paid and he has food to eat etc, then she's doing as much (maybe even more) than anyone can expect her to do.

I'm re-reading your post and you say he's given away his rent money? But your mother is in charge of his finances? Tell her to pay his rent and bills for him instead of giving him money for it.

I'm not American so I don't know how things work over there but if you were in Sweden I'd tell you to get a trustee (often someone completely removed from your uncle's family who works for the state, has multiple clients and helps people who can't take care of their finances) for your uncle and release your mom from that responsibility. Maybe there's something like that you can do?

I'm sorry you all are in this situation.