There will not be information to validate your concerns since the OraQuick site itself provides information that testing for validity has not been done on children under the age of 17.
The obvious answer to attempt to alleviate your continuous concerns over this non-event is a blood test for your son (please understand that YOU, not he needs this test to ease your worries).
However, since you have posted that your ex-wife must (it appears) give permission for such testing..and refuses to do so...there is really nothing else this forum can do to help you deal with your continuous focus on this issue.
I get cold sores. My immune system is perfectly fine and I certainly don't have HIV. In fact, the majority of humans carry HSV (the virus that causes oral cold sores), having picked this up in childhood.
These things have nothing to do with HIV because testing has clearly shown that your son is HIV negative.
Of course, as I write this I realize I am just wasting energy doing so.
How would you propose that the virus gets "started"? It just doesn't show up out of thin air.
Again, in order for you to become infected you would need to come in contact with the bodily fluids (i.e., sexual fluids, blood) of an infected individual and those bodily fluids must be able to get INTO your own body.
Why was this incident not risky for you?
1. Damaged skin near a nail is not an open portal into the body.
2. And more importantly, said girlfriend doesn't have the virus in the first place. She has nothing to pass onto you. Simply touching her menstrual blood even with damaged skin doesn't make HIV suddenly appear out of nowhere.