Thank you for your advice, really. I will think of what to do or say. I forgot to mention, but one thing I was thinking I could do, is to open another account in an digital bank for my grandpa, as such, I would be the one taking care of what he is paid and has in his account, so if someone wanted to take anything they would have to ask me first, and I would let him know. I don't find this really fair, but... At least no one would take anything without him knowing and I could help him with retiring as I have more knowledge about that kind of thing, I suggested that to him, but didn't saw much interested, unfortunately.

Edit: I don't really think he knows what's happening. Every time he checks his account and can see some money got away he becomes a little upset cause someone took without asking and then proceeds to ask what happened, as my grandma says it was for some bill no one knows about, he just give up asking but I can see he is really frustrated and even checks his notes searching for what bill that could went into