The story is related about a father whose son was becoming increasingly disinterested with Yiddishkeit and moving ever closer to going off the derech. He went to seek the advice of the Baal Shem Tov, who upon hearing the man's plight, told him simply to 'love him more'.
I have seen this story in multiple places (here, here and here) and is regarded as a very powerful tool in Kiruv (outreach), a way of approaching those who are disengaged with Judaism and seeking to leave the fold.
Does anyone know if this story is related in any seforim as I would like to quote it from a proper source. I appreciate, given the period that the Besht lived, it would be a much later rabbinic source.