He is the eldest son and thus heir to the title.

他是长子，因此是这个封号的继承人。

《牛津词典》

He made over the property to his eldest son.

他把财产传给了他的长子。

《牛津词典》

Ji Zha refused because he thought the king's eldest son should be the one to receive the throne.

季札拒绝了，因为他认为应该由君主的长子来继承王位。

youdao

He says his eldest son has just graduated from university in New York and has spent some time working alongside him on the business in Cameroon.

BBC: Housing Cameroon's diaspora

But having decided to be the dutiful eldest son he set about transforming the family business into something beyond the confines of 1920s Dallas.

ECONOMIST: Stanley Marcus

He also just happens to be the eldest son of the Conservative Party's agriculture spokesman, Tim Yeo.

ECONOMIST: Canvassing the politicians