I think it depends how much you want to be involved.
Reading between the lines I would suggest something has happened for his behavior to change. Either he has lost a contract and has more time at home or something in his personal life
I think there is no harm in just asking 'are you alright mate, you seem to be drinking a little earlier than I am use to'. I think sometimes, people just want someone to take an interest in them and if it just a question of their well being it is not seen as prying. Even just a little question that may lead to him reaching out to you in a few days after you open the door of 'care'. If he spends so much time alone then he just may not have any interaction.
I do think also telling his brother is a great idea as well.
It's a tough one. I've asked him to do a few things lately but he doesn't seem up for it. That's not very strange as he has always been like that. If I, or our friends, were doing something he'd often say he wasn't up for it.
I saw nothing out of the ordinary until my missus actually said it to me about bumping into a couple of times and him seeming drunk. Him staying home was always just natural, but when she said it I took more notice. When I asked him was he OK and if he was drinking a lot he insisted he wasn't. I didn't want to probe too much in case it forced him away from me. I've made a more conscious effort to keep in contact with him.
He definitely hasn't been at it all those years though, so I think you're right, something has changed.
He's a lovely fella and I'd hate for him to be going through something alone.
