Flexibility work will definitely help his tennis game. Strength resistance training should also be a part of his overall program. Since he’s intimidated by the weight room, can you train him in another environment? And if weights are intimidating, start with bands, tubing or body weight resistance. His fatigue may come from his response to the environment, a previous experience with strength training or a lack of having strength trained before. Just because he’s got cardio endurance on the court, doesn’t necessarily translate to strength endurance.