"You are my boy."
What if this is not a child? Could an adult say this to a grown man? I say this to my male friends all the time in English when they do something agreeable. I need something they produce it I will tell them "YOU are my boy!" give them a high five or something....maybe? Or maybe my favorite waiter?
'Vous' forms can be plural or they can be polite-singular. Since the example says 'my boy' (singular), one can use 'tu' or 'vous' to express it in French. My argument is that you'd never. IRL, use a polite form of address in the semantics of the situation: any speaker who speaks this particular sentence will be familiar enough with 'my boy' to use 'tu'. So, grammatically, one can use 'tu' or 'vous', but since the object is singular in the example, you cannot pluralize /boy/ into /boy's'/. Practically, though, I doubt one would ever use polite-vous in this context.