Herpes is quite hard to avoid, I think. It is very common and hard to prevent transmission. See below:
'How common is herpes simplex?
Very common. By age 25, about six out of ten people in the UK carry type 1 and about one in ten carries type 2, more in the sexually active population. If this surprises you, it is because most people who have it don’t know that they do, because they have no symptoms or because they get it so mildly, they do not notice.'
herpes.org.uk/frequently-asked-questions/
If you have ever had cold sores in the past and orally go down on your partner's genitals, you are exposing them to herpes (HSV1). You don't need to have an active cold sore either for it to be 'shedding'.
Also those people who have sex with a partner based upon them having a recent clear STI check do not have protection against contracting herpes because the test does not reveal it unless active sores are present and tested. You'd need to get a blood test to rule it out. This is not offered as part of the standard STI tests, as far as I am aware.
Mrs Pringles - According to what I have read, it is most contagious when active (blisters/sores) but is still transmittable during periods when it is shedding, which is invisible to the naked eye. Some people contract it and never have any symptoms!
goaskalice.columbia.edu/answered-questions/shedding-light-viral-shedding
Of course, if a person knows they have it then they should be open with a partner about it, but I don't think it is easy to avoid/prevent transmission even if the partner is told about it. Some people use herpes dating services to meet other people who already have it (especially people with HSV2).
OP - Perhaps you could do it like this... When he says how are you or how have you been or something like that, tell him that you haven't been too good and say, in fact, it is something I need to talk to you about. I contracted herpes (years ago?) and have not had an active outbreak since I met you. To be honest, I had forgotten about it. I recently had an outbreak and I realised when this occurred that I have been extremely remiss in not having informed you before we slept together. I am very sorry.
Perhaps print some info off the herpes site about it. They have advice on how to deal with partners and it. You can make him aware of how common it is and transmissibility etc. If he knows nothing about it, I would prepare for him to react badly as many people are unaware of how prevalent it is and how likely they are to have been exposed to it previously with knowing it. I have to admit, I'd be very upset if this happened to me (as the person not told), but I do understand how it's happened. I think when it's inactive you tend to think of it as being gone, when it's not really.
He also made the decision to have unprotected sex with you so he should accept that it's not without risk, regardless of prior testing.