I was thinking about this very thing on Friday. No, I tell a lie, it was Thursday, just before my doctor's appointment. No, sorry, it must have been Friday, because I remember I bought my lottery ticket just before it happened and I always buy it at the same time at the same shop. I needed some milk as well as your Auntie Edna was coming for a cup of tea at 11am (well, and a slice of that cake she loves too) - although it was nearer quarter-past as her bus was held up in traffic where they're building the new housing estate and they've got temporary traffic lights. Anyway, we always have semi, but she can only drink skimmed, because of her heart, and the shop had just sold out of skimmed, so I had to try the new shop across the road. It was the same price as the other shop, but they'd had an offer on if you bought two, so I was a bit peeved at that. Have you been to that new shop? It's owned by the son of a lady I went to school with. She always got into trouble at school - I remember one time when she.........
Oh, I feel your pain, OP. Thankfully, the person this is redolent of isn't a member of my immediate family, so I see him regularly but I don't live with him - although he is really lovely.
As PP have said, it's partly a lack of internal monologue and personal filter, but I think it's often just that they don't have enough to occupy their minds, so they really concentrate on the petty, insignificant stuff. It might also suggest a degree of loneliness and by keeping a conversation going (albeit one-way) for ages, they get that human interaction. Sadly, though, they have to work with whatever material they can summon up and Dorothy Parker or Oscar Wilde they are not.
Does he have hobbies or responsibilities that occupy his time and his mind? Does he meet many people day to day? I think you might just have to accept that this is how he is and learn to zone out when he shows signs of cranking up to a long, pointless monologue. Not wanting to be cruel, but sometimes reacting a bit cool and detached can help as going along with it and acting interested just tends to add more power to their elbows.
If you do want to go down the accepting-no-nonsense-light- confrontation route, you could do much worse than taking tuition from Winston in his dealings with Isa in Still Game. My favourite of his tactics is to extend his arm and hold out his flattened hand towards her, bobbing his thumb up and down frantically with a pained look on his face whilst she gabs. When she realises what he's doing and breaks off to question it, he tells her that he's trying to work out how to fast forward her to the relevant part 