After completing his service, he chose to re-up for three more years.

re-up 是由前缀 re-（表示"再次"）和动词 up（这里指"入伍、参军"）组成的复合动词，字面意思是"再次入伍"。在军事用语中特指"延长服役期限"或"重新入伍"。