I don't know 「what has become of him since then [what became of him after that].(▼英語では動詞の完了形で「その後」の意味を表すことができるので,必ずしも訳す必要はない.またafter thatには過去形,since thenには現在完了形を用いるのが普通)
橋は台風で壊れたが,その後新しいのが架かった
The bridge was destroyed by the typhoon, but a new one 「has been built since [was built later].
その後数日たって彼は危篤状態になった
A few days later his condition became critical.
その後数日は頭痛がした
I had a headache for several days afterwards.
その後皆様お元気でいらっしゃいますか
〔手紙の文章〕How are you all getting on [along] these days?
その後の情報は入っていません
No further news has been received.
市の産業はその後めざましい発展を遂げた
The subsequent development of the city's industries was remarkable.