Wow, I'm really sorry that this is happening. He should have told you.
It is possible for him to have had it this long without you getting it.
If all you do is avoid sex when he has symptoms, you have a 10% chance of getting it per year, assuming a monogamous relationship and sex 2-3x a week.
If he takes daily antiviral medication, like Valtrex, OR you use condoms, it reduces those chances to about 5% a year.
If you use both of those things, it's about 2-3%.
Obviously, those are statistics, and don't begin to cover things like trust. He made a decision regarding your health for you.
Do you have children? If you had gotten infected during pregnancy, that could have caused serious harm to your babies.
I say this as someone who's had hsv2 for 20 years - he should have told you from the start.
I can't tell you if he's had it the whole time, or if he's trying to cover up cheating. No blood test would confirm that, either. Did he get a type specific IgG blood test? That can only tell you that you've developed antibodies, and you can develop antibodies in as little as a few weeks.
Did it give him an index number? Sometimes, if it's a lower number, that means it's a newer infection, but that's not a hard rule, either. If this is the only reason you'd suspect him of cheating, it's not enough to prove it.
I'm really sorry. I can't tell you what to do from here - not my marriage. I can tell you that if you're upset and/or mad, you have rights to be.