There really is no way to know how long either of you have had it if the numbers are definitively in the positive range. I take it he was also positive for HSV2 and that these were both IgG herpes tests?
If you are both positive for type 2, you can't pass it back and forth now, so you won't have any restrictions on your sex life. However, if you are bothered by outbreaks, you can always use suppressive medication or take medication at the time of an outbreak, just for your comfort level.
I personally see no point at this stage in trying to find out how long you have both had HSV2 fact is you have it now and you should both concentrate more on how to manage it rather than beating youselves up on timing which you will most likely cause more upset than the HSV2 and someing you may never find out as many people display no symptoms for a very long time,and some not at all,hence the reason many people don't even know they have it.
The risk of contracting HSV2 begins as soon as we become sexually active and indulge in unprotected sex whether it be with one person or more,the risk is always there.