Wow, it sounds like this is a very tough situation. Generally speaking, one incidence of oral sex with a woman is not very likely to result in HIV transmission. Nevertheless, he should be tested by a doctor
at least once.
Assuming that he still believes that he has HIV or could have HIV, he should definitely be examined and interviewed by a mental health care professional. I cannot tell from your question if he has OCD, or another mental health condition such as a psychotic disorder (ex. schizophrenia) with obsessive features. I actually think the second is possible, but less likely. Either way a combination of medications and behavioral therapy is needed to overcome these obsessions if that's what they are.
I think this issue is best handled by a psychiatrist
, after he has an in office HIV test with his primary care physician
. He or she can prescribe the right medications and begin him on the right type of therapy. Most likely, he would benefit from an SSRI and cognitive behavioral therapy, which is a good combination for obsessive compulsive disorder. If the psychiatrist finds that these are not obsessions, but rather delusions, a different therapy will be needed.