Forgive me, for when it comes to Pokemon I'm a complete novice outside of getting it for my brother for Christmas whenever a new version comes out. However, I'm feeling particularly dumb when it comes to this game, but I know it's something he'll want.
This is a partial remake of the original games, which I get. But when it talks about being 'tightly integrated' with Pokemon Go on mobile what exactly does that mean? Would he need to have separate account and the game on his mobile to play, and does it mean that he'd be having to pay additional real world money for resources like on mobile?
You can play completely Let's Go without having to interact with Go. About the "integration", I think it refers to the fact the a lot of features in Let's Go are similar to Go - first of all the catching phase, the management of the Pokémon box etc. However the actual interaction isn't necessary, though Go you're able to get Meltan and its evolution, but they are just an extra. Let's Go is a self-contained experience (and for your brother I'm sure it will be a really fun one).