this is definitely something to talk about face to face. Before you initiate the conversation, you would want to make sure you know what sort of herpes is it that you have and what are the treatment options for it. Is it the cold sore herpes which is contagious and can remain dormant in the body, but it is not an STD or is it something else.
You can start by telling him that you've had a cold sore and you know that they are highly contagious. Ask him if he had had any problems himself with that and then take it from there.
If the herpes outbreak is not sexually transmitted, then there won't be any point at this time to talk about that.