Encourage your candidate to engage in outdoor activities such as sports or cycling to work.
Evaluate with your candidate how he organizes his life and deals with potential problems. Does anything require a substantial amount of energy?
Evaluate with the candidate what kind of work costs him a lot of energy and what kind of work gives him energy. Try and find a balance between the two.
Evaluate with the candidate what activities give him energy in his spare time. Encourage the candidate to engage in those activities.
Investigate with your candidate what could cause him to feel less energetic. Can the cause be found in the work’s content or its organization; in colleagues, or personal issues?