Hi OP,
I'm sorry this is proving stressful for you and your son. I can speak to this as I had a positive test for HSV1 genitally one month ago and my god is it draining, upsetting, overwhelming etc especially when all you've ever heard is the stigma around herpes. This is a bit of a long response but I've tried to share what the last month has taught me as I know how horrible it is when you first find out.
Does he know if it is HSV1 or HSV2? That will make a bit of difference to the likelihood of reoccurance etc.
First of all, it's really not a big deal. I know it will be hard reading that and you may disagree at the moment. I spent the first two weeks crying, hyperventilating, panicking about the future, feeling dirty, and not knowing what to do.
The first outbreak is said to be the worst, I found walking and sitting down painful so you can imagine how that only deepens the spiral of 'ahhh it's going to be like this forever!'.
The sexual health clinic that I went to recommended the Herpes Virus Assosication (HVA) and strictly nowhere else for advice so please don't take any of the stats shared here or on any other forums/sites as gospel. The initial panic sends you straight to Google or ai where you'll find horror stories but for most people it's a non-issue after the first occurence. I devoured the HVA website and it made me feel so much better, I also rang their helpline which was the turning point for me going from sobbing daily and thinking my life was ruined to thinking 'ahh well, it's a pain but most people have the virus, it's just an unfortunate inconvinence much the same as a cold'.
I'm in my 30s and all sexual health professionals so far have said that by this age it's 50/50 whether people have it or not and 60-80% (depending on which sources you refer to) of the UK population has HSV1 orally or gentially.
There are actually a few positives (I'm not going to share potential misinformation that I've misremembered with you, but if he rings the helpline tell him to ask if there are any upsides with regards to antibodies etc).
The clinic also recommended a YouTube video by Ella Dawson: 'STIs aren't a consequence, they're inevitable' - very helpful and reassuring video. Please do watch it!
Finally just to say that this isn't a consequence of him being irresponsible. I am an extremely responsible adult and have had two partners in my life, both times we have been fully tested before sleeping together and had conversations about contraception etc. HSV isn't part of standard tests because it's assumed most people have it anyway and as others have said, you can still catch HSV even if you use condoms.
My partner had no idea he was an asymptomatic carrier. He has been incredible and the sense of guilt he feels is pretty heartbreaking for me to watch. I'm not sure if it's worse to be the giver or the receiver in this case! Hopefully this is a big wake up call to his girlfriend to be upfront about it in the future.
I'm at the tail end of the symptoms now but I honestly thought they would never end a couple of weeks ago, it's like having chickpenpox on your bits which makes trying to forget about it pretty difficult, but it will heal and if it ever reoccurs it will be milder.
The most useful thing I found was Vaseline to prevent chaffing against underwear and clothes when you go out. And not wearing underwear at home and wearing loose clothing. HVA also sell LomaHerpan coldsore cream on their website which I have bought for if I feel a prodome (telltale tingle in the area) in the future. I am also taking 1000mg of L-Lysine twice a day in the morning and before bed as this is said to help - but as I've said do your own research into that and don't believe anything you read online other than from official HVA or NHS sources.
I hope your son heals quickly and that his mood and mental health don't take too big of a hit. It's a pain but in all honestly if he didn't catch it now he would likely have caught it at some point in the future. Lots of people, (roughly 2 in 3), have HSV1 and will never know because they have no symptoms or the symptoms are so mild they think it's something else.
Big hugs to you both xx