He is highly skilled, but in his current work place, he is unable to put his abilities to use.
For this reason, he is unmotivated and feels unfulfilled with his work.
ご質問ありがとうございます。
"He is highly skilled, but in his current work place,"＝「彼は非常に能力があるのだが、彼の現在の職場では」
"he is unable to put his abilities to use."＝「彼は自分の能力を活かせない。」
"For this reason, he is unmotivated and feels unfulfilled with his work."＝「その理由で、彼はやる気が無く、自分の仕事に充実感を得ていない。」
ご参考になると幸いです。
He can't use his abilities to their full potential, so he's stagnating at his current job.
「今の職場で彼は、自分の能力を活かせなくて、腐ってる」は英語で「He can't use his abilities to their full potential, so he's stagnating at his current job.」と言います。「腐っている」は普段英語で「rotting」と翻訳されますが、仕事で思い通りにいかなくて、やる気を失ってるというニュアンスなら「stagnating（成長せずにそのままで）」のが適切です。
今の職場で彼は、自分の能力を活かせなくて、腐っているので転職を検討しているらしいです。
He can't use his abilities to their full potential, so he's stagnating at his current job, but I heard he's thinking of changing jobs.
アメリカ合衆国 
カナダ 
