This is generally understood to refer to Lloyd Osbourne, Robert Louis Stevenson's stepson. Stevenson often referred to Lloyd as "Slo" as a nickname, short for "S.L.O." which stands for "Stevenson Lloyd Osbourne".
Though the dedication is a simple phrase, it carries a deep meaning. Stevenson had a close and complicated relationship with Lloyd, who served as his collaborator and confidant. He saw Lloyd as a source of inspiration and a kindred spirit, and dedicated the book to him as a sign of their strong bond.