I wouldnt tell until you were comfortable. You don't want to scare him off. After he gets to know you then you can tell him. Most BPD suffferers don't exhibit symptoms when they first start dating someone. here is a good webpage to show how you change during the dating process. How a Borderline Personality Disorder Love Relationship Evolves - Roger Melton, M.A.
This was true for me. Although when I started dating my fiance I wasnt diagnosed with bipolar or BPD yet but I knew I had issues.