。
He has the caliber to become a future pillar of the nation.
Literal
He [topic-は] future nation [possessive-の] leader [classical copula-たる][should-べき] caliber [copula-だ].
棟梁 'pillar, leader' (literally 'ridgepole') is used metaphorically for a national leader. たるべき is literary: たる is the classical attributive copula (modern である) + べき 'should.' 器 means 'vessel, capacity, caliber' — the metaphor of a person as a container of ability. The whole sentence has a grand, literary quality.