Boromir would have acknowledged that Thorin Oakenshield is a King approximately 10 seconds after meeting him.
He would’ve immediately volunteered to come along to the Quest, because the military situation in Gondor is manageable right now, and besides they really do not need the possibility of a dragon allying himself with Mordor, seriously, orcs are bad enough.
Boromir then proceeds to appoint himself as the official protector of everyone around him (because that’s what he does) and after a while he starts to challenge the people who sound like they’re being less than respectful towards Thorin, because fuck that. From what he’s seen and heard, this guy has been fighting tooth and nail for his people for who knows how long, so hold your tongues you’re talking to a King.
And the Company is just staring at Boromir, because there’s no way they’ve somehow managed to find a human Dwalin. Not possible. But there he is, asking those Men if they’d like to have Gondor declare war on them, because that can be arranged, and oh Mahal, definitely a human Dwalin. How???
Thorin isn’t exactly sure what’s happening or why, but this Lord of Men is defending dwarven honour and there’s no way he’s ever forgetting that. Boromir doesn’t know it yet, but he’s just accidentally secured an alliance between Gondor and Erebor.
Dwalin is nearly beside himself with approval.