Translation:The man is strong, the boy is weak.
Both are in Nominative. Sometimes the hints create confusion rather than help. "silny" can be also Accusative, but not in many contexts, as here the notion of being animate or non-animate comes into the picture: animate would be "silnego", non-animate would be "silny" (Wieje silny wiatr = Strong wind is blowing).
In "to jest" structure, which is not here, Nominative is used. Instrumental is an option for X is Y sentences (X in Nominative + a form of "być" + Y in Instrumental), but that applies when Y is a noun phrase. If it's just an adjective, Nominative is used.
It is not. It's of Slavic origin.