As long as your father has capacity and from your description it sounds like he does and backed up by the fact there is no formal diagnosis he can simply cancel the LPA (link to the UK Gov website below). If he had the capacity to set it up as little as 6 weeks ago then there's not too much argument that he's gone downhill in that short a space of time so he can if he wants change it to someone else or add another person.
The family politics of who tell the son, what they tell him and why it's happening is something else.
Firstly you'll have to get Dad to agree and it may be that may be a problem, the son he sees everyday (possibly) versus the daughter half a world away, would he be prepared to do it?
It might be your Dad helps the son out a bit (financially) for all the help he gets from him on the "you can't take it with you when you go" basis so dishing out a few quid to your helpful son isn't a problem to him, if he's getting some money from Dad it might be with his consent, it's possible!
It'll need some very delicate handling and it's a bit of a no win situation either way someone is going to fall out. Chuck in the alcohol issue too light the blue touchpaper and retire to a safe distance as it says on fireworks.
If you can show the facts to your Dad that your brother is taking his money without his consent then they fall out, if you can't convince him and he sides with your brother then you and him and your brother all fall out, serious possibilities of a mega family rift.
Sorry if that doesn't sound too helpful/hopeful they're just my thoughts for what they're worth.
K