Obviously, your best bet is to do it BEFORE you have any sort of sexual contact, but since that ship has sailed, let's try to do some damage control:

1.  Just say it.  Be blunt.  Men appreciate it in the end.  Do it sooner than later, too.

2.  After giving him the initial news, make sure he knows that just because you two have had sexual contact, that it doesn't mean that he DEFINITELY has it.  Reassure him that it's not written in stone that you WILL contract the virus by having sexual contact with someone that has it.  Then, make sure he knows that there is still a risk that he MAY have it, and that he should get tested.

3.  Let him know about your outbreak areas... even though you can still spread the virus without an outbreak, if he hasn't come into contact with your outbreak areas, it minimizes his chance of catching it.

4.  Apologize, apologize, apologize, and be ready for any misdirected emotions that he might throw at you.  This wouldn't even be a step if you had told him in the first place.  If you had, then he would have known that he was proceeding at his own risk.

5.  Encourage him to get tested so that he doesn't spread it if he has it.

After that, you've done all you can.  I've found the best way to help people be comfortable with it is to give them as much knowledge as possible.  In reality, herpes is nothing more than an annoying skin condition, really... you can still have kids, have sex, etc.  BUT, your potential partners deserve to know, so that THEY can make the decision of whether or not to have sex with you.

You need to be prepared for possible rejection and a broken relationship with this guy.  I hope that he's understanding and forgiving, but his knee-jerk reaction will probably be painful and feeling like he's been deceived.  Again, avoid it in the future by being open and honest with people beforehand.