Encourage your candidate to ask others for 360º feedback regarding his personal development. What goes well and what could be improved? Discuss the results with the candidate.
Investigate with your candidate what could be an interesting interaction with another person. Ask him afterwards what went well and what he found difficult when interacting with the other person. Ask him what could be improved and how, and practise with him.
Encourage your candidate to experiment with behavior that is opposed to his normal behavior. Practise such situations together first.
Ask the candidate to gather new, recent information on a topic that is important to him. Ask how this has changed his view on the topic. Ask if and how he can apply this information in his work. See if brainstorming on this question works.