It is definitely possible that you have genital herpes and that your partner does not. Only your partner knows for sure if he is telling the truth, but here are some scenarios in which he could be correct about not having herpes when you do. One possibility is that you have had herpes for a long time and were just recently diagnosed. Many people with genital herpes are unaware that they are infected and find out when they get a positive blood test for herpes, or when they develop symptoms of an outbreak. It can be very hard to accurately determine when you contracted a herpes infection. The only way to prove that your current outbreak is new is to have a swab test of the genital lesions turn out positive right now, and a blood test for the same type of herpes be negative right now. If a second blood test a few months later turns out positive it would confirm that this is a new infection, since blood tests take a few months after a first herpes outbreak to turn positive. If your herpes blood test is positive at the time of your first outbreak, this tells us that you have had that type of herpes for at least a couple of months, but we cannot be any more specific about when you first got the infection. It is also possible that your partner does have herpes and had a false negative test. Herpes blood tests can be negative even though someone is actually infected. This is most common in the first few weeks after infection, before the body has produced antibodies to the virus – the blood test is looking for these antibodies, so the blood test can be negative during the first few weeks after someone has been infected with herpes. Additionally, herpes does not transmit 100% of the time. It’s possible to have a long-term partner who has herpes and to never get the infection.
If your partner does not have herpes, there are some things for you both to know to move forward. If you have herpes, it is not a guarantee that you will give the infection to your partner. One study showed that transmission between couples is about 10% per year, and about 70% of these transmissions happened when the infected partner had no symptoms (https://annals.org/aim/article-abstract/705283/risk-factors-sexual-transmission-genital-herpes?volume=116&issue=3&page=197).
Condom use can reduce (but not eliminate) the risk of transmission. One study showed that using condoms most of the time reduces transmission from an infected man to an uninfected woman (https://jamanetwork.com/journals/jama/fullarticle/193953) – though this study only looked at monogamous heterosexual couples. Another study analyzed multiple other studies and found that people who use condoms every time they have sex have a much lower risk of contracting HSV from an infected partner (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2860381/).
Daily medication (using either acyclovir or valacyclovir) can reduce the risk of you spreading the infection to a partner. One study of people with genital HSV-2 who took a daily antiviral medication showed that people taking medication were both less likely to shed virus, and less likely to transmit the virus to their partners (https://www.nejm.org/doi/10.1056/NEJMoa035144?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dwww.ncbi.nlm.nih.gov). In addition, people who both took daily medication and used condoms for intercourse had no cases of herpes transmission in this study.
With herpes, as with any STI, a diagnosis shouldn't be about the blame game. What it should be about is good communication between you and your partner(s), education about the disease and the best way to stay healthy, and limiting transmission to others in the future. Herpes is incredibly common, but very manageable.