He was really gaunt in his last few months from being so sick.
この場合、次のような表現ができると思います。
ーHe was so sick and thin in his dying days.
「彼は晩年病気でとてもやつれていた」
ーHe was really gaunt in his last few months from being so sick.
「彼はとても具合が悪く、最後の数ヶ月はかなりやせ衰えていた」
例:
He looked like a completely different dog/cat compared to when he was healthy. He was so sick and thin in his dying days.
「彼は元気な時に比べると別の犬/猫のようだった。彼は晩年病気でとてもやつれていた」
ご参考まで!