"He was so sedentary from studying for entrance exams that he was completely out of shape."
"He was so sedentary from studying for entrance exams that he was completely out of shape."
"he was so sedentary" 「彼は座りがちだった」
"from studying for entrance exams" 「受験勉強のため」
"he was completely out of shape" 「すっかり体力が落ちてた」
Because he was sedentary from studying from entrance exams, he's become out of shape.
ご質問ありがとうございます。
「彼は」=「he」
「受験勉強で」=「from studying from entrance exams」
「座りがちだった為」=「because (he was) sedentary」
「すっかり体力が落ちてた」=「(he's) become out of shape」
「彼が座りがちだった」を強調するために、「sedentary」の前に「so」=「とても」を使っても大丈夫です。
「become out of shape」の代わりに「easily runs out of energy」もよく言われると思います。
ご参考になれば幸いです。