I am a mother of nine children. One of them, has always been aside whenever we are busy doing a mitsva (Kiddush, Havdala, etc.) He is now 13 and is expressing his lack of Emunah. What should we do?

There is not one answer that can fit all children and all families. Since, however, you describe the problem as having been present "always" - before he was of an age to have even the beginnings of a mature understanding of Mitzvot - it is likely that the problem relates not to Emunah [= Jewish faith] but to his place in the family and/or other social settings. Check his behavior in school, let him know how seriously you see the performance of Mitzvot, and make it clear that his lack of participation is not an option. Depending on the circumstances you may choose to consult a professional in family guidance. B'hatzlacha!