Figure out what your candidate's standards for his own accomplishments are. Is he more ambitious in certain kinds of work than in others? In what kinds of work? Ask him to find out how he could engage more in that kind of work.
Ask your candidate to think about opportunities for himself and ask how these could be realized.
Discuss your candidate’s personal future goals. Confront him, when necessary, with a lack of ambition and ask where it comes from.
Make sure your candidate is engaged in work in which he is successful; this might encourage the person to achieve more.
Review what kind of work brings out ambition in the candidate most and least. Discuss these differences and try to figure where they come from.