Hi OP - as a sufferer who take aciclovir, I just wanted to add a couple of things to the thread.
From the WHO site: "An estimated 3.8 billion people under age 50 (64%) globally have herpes simplex virus type 1 (HSV-1) infection, the main cause of oral herpes. An estimated 520 million people aged 15–49 (13%) worldwide have herpes simplex virus type 2 (HSV-2) infection, the main cause of genital herpes."
Both strains of the virus can cause coldsores and/or genital sores, it is just that 1 is more likely to recur on the mouth, and 2 is more likely to recur on the genitals.
The younger a person is when they contract the virus (excepting newborns and babies for whom it can be very dangerous) the more antibodies they will build up, therefore reducing the likelihood of recurring outbreaks. This is why some kids who had coldsores rarely/never get them as adults.
I also read that only 25% of people who contract the virus show symptoms (can't remember where, NHS Choices is a good source of info). The most definitive test is a blood test for antibodies, but this cannot tell you where/when/from whom you contracted the virus.
Between outbreaks (if one has them) herpes lays dormant in the spinal column, in the nerve that it originally infected. This is why it is recurrent in the same spots for sufferers with ourbreaks.
Aciclovir can be taken to manage active outbreaks, or as a supression therapy (although I think this is usually in the early days after diagnosis and limited to 6 months).
He might also have used an online pharmacy for this specific medication because it is downright embarrassing to go to a doctor or to stand in a pharmacy and effectively tell a stranger "I have genital herpes".
You are clearly thinking this through before talking to your DH, but FWIW, a herpes outbreak alone does not indicate that your DH has been unfaithful.
However, I would have questions about how long he's had it, how long he's known, and if he knows where/how he got it. There is nothing wrong with carrying a virus, but there is absolutely something wrong with not informing your intimate partner about a diagnosis that could impact their sexual health.