Welcome back. I'm sorry to hear the diagnosis was confirmed, but obviously not surprised.
Almost certainly this is your first HSV infection. It could be either HSV-1 or HSV-2; the test from the swab probably will tell, but if not, the blood tests will. (If both swab and blood are negative, you'll need to have another blood test in a few weeks, expecting it to become positive to one type or the other.) In other words, probably your boyfriend is infected. Up to half the population (in the US) has HSV-1, usually orally, often without symptoms -- so in that sense, your partner might not have an STD at all. Or he could have an asymptomatic genital HSV-2 infection, from some past partner. Up to 20% of all people have HSV-2, and most of them also have no symptoms.
As for figuring out "what to say": Just be straightforward, calm, and non-accusatory. He'll probably be as upset as you are. As it happens, I just addressed this issue in more detail in a new question that I answered just a few minutes ago. Please read that thread:
http://www.medhelp.org/posts/STDs/vaginal-sore/show/1108188
In addition, use the search link for lots of other information (try entering terms like "initial herpes"). Also take advantage of MedHelp's excellent herpes community forum, where the moderators are genuine expert herpes counselors. You'll find many discussions about talking with partners. Finally, try looking at the information available from the American Social Health Association (www.ashastd.org) and the Westover Heights Clinic of Portland, OR (www.westoverheights.com). (Full disclosure: Dr. Hook and I serve on ASHA's Board of Directors, and WHC is run by Terri Warren, who moderates MedHelp's herpes forum).