。
I'm just a country boy now.
Literal
I [topic-は] already countryside [possessive-の] boy [explanatory-なのだ].
もう ('already, now') signals a change of state — the speaker has become a country boy. 田舎の少年 ('country boy'). なのだ is the explanatory copula — the speaker is declaring their identity with emphasis. Context suggests either a life change or a wistful acceptance.