When someone says his HIV is undetectable, he is referring to the viral load. Because the assay cannot detect viral load, does not mean there are no viral particles in the blood. The number of viruses may be so few that the assay cannot detect them.
Viral loads are usually reported as copies of HIV in one milliliter of blood. The tests count up to about 1 million copies, and are always being improved to be more sensitive. The first bDNA test measured down to 10,000 copies. The second generation could detect as few as 500 copies. Now there are ultra sensitive tests for research that can detect less than 5 copies.
Only about 2% of the HIV in your body is in the blood. The viral load test does not measure how much HIV is in body tissues like the lymph nodes, spleen, or brain. HIV levels in lymph tissue and semen go down when blood levels go down, but not at the same time or the same rate.
Taken together all the recent reports continue to suggest that effective combination antiretroviral therapy that drives HIV plasma viral load to undetectable levels significantly decreases the risk of HIV transmission. However, the risk is not nonexistent. One of the issues recently getting more attention is the now confirmed fact that even though the plasma viral load is suppressed to undetectable limits, folks can still have detectable virus in seminal fluid (and presumably anal secretions and vaginal/cervical secretions as well).
There is no "safe" level of viral load. Although the risk is less, you can pass HIV to another person even if your viral load is undetectable.
The risk of HIV transmission from oral sex, if a person has an undetectable viral load, are extremely low.
You should discuss the situation with you family physician or HIV clinic. HIV testing will be one of the recommendations. You should learn everything you can on HIV.http://www.aids.org/factSheets/125-Viral-Load-Tests.html