Hugo
I'm sorry you have this to worry about. The fact is that a majority of adults have oral herpes, although most are asymptomatic. Most of us who have it, got it as a child from exactly the scenario you describe. If he gets it, it's not the end of the world, but obviously you'd like to avoid it if possible.
I'm not sure why you are so against using acyclovir as it has a very long record of being very safe while the effectiveness and safety of "alternative" or herbal remedies is questionable. Acyclovir and valtrex are now very commonly prescribed, in the third trimester, for pregnant mothers who have herpes to suppress possible outbreaks at delivery so it's considered to be very safe. If your son does begin to display symptoms, I'd recommend getting him to a doctor as soon as possible so that antiviral meds can be started ASAP as their effectiveness is much greater before an outbreak gets going full force.
Let me know if I can help further. You might be able to discuss this with some nurses who participate in the Herpes Home Page forums at
http://racoon.com/cgi-bin/dcforum/dcboard.cgi
Disclaimer: I am not a doctor nor am I medically trained.