How is he physically? If he’s not frail, maybe he could do volunteer work: holding preemies in the NICU is one thing older people do.

Maybe let him donate a certain amount to legit charities

Giving money to scammers is fulfilling his need to feel useful and do good. He needs a more productive way to do that.

Also get the phone under control. I know there are special phones for seniors that have ways of blocking scammers.

Does he know they are scammers? You use the word “habit.” What does he get out of it?

Having someone declared incompetent is complex, and you need a lawyer.

In the meantime could you swipe his phone and get the scam numbers blocked on it? It seems like stopping him from even receiving the calls is key.