I have had herpes for 30 years and take Valcyclovir  500 mg every day to control asymptomatic shedding. My partner  removed his condom during intercourse last night for about 20 minutes.  He  showered immediately after.  Is there any preventative or protective measure that he could do or take for  possible exposure?

Good morning.
No, I think you both did everything possible to avoid transmission here.  I'm assuming he knows that you have herpes, is that correct?  Do you know if this partner has ever actually been tested to know if he might have herpes and not know it?  This is something I encourage in couples who believe that they are discordant - that is one person is infected and the other is not.  If he happens to be infected, you can stop taking medication and he can stop using a condom to reduce the changes of getting your herpes (though he might want to use it for other reasons, I don't know). 
If he is actually negative, you are currently doing all that you can to reduce the risk of him getting herpes.  Please let me know if you have other questions.

Terri
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Yes, he knows. If he wants to understand his chance of contracting herpes in this situation, could you estimate a percentage chance of transmission, when I am taking the Valcyclovir daily and am experiencing no symptoms? Also, what percentage of the population has herpes and is unaware because they never had the initial outbreak or symptoms? 

Yes, of course.
With him using condoms and you taking daily antiviral therapy and avoiding sex with outbreaks and him being aware (and assuming he is not infected) and having sex about twice per week, the risk of him acquiring HSV 2 from you is about 1% per year - that is about one male in your situation out of 100 would acquire herpes.
About 80% of the population that has HSV 2 doesn't know it.  This may either be because they are missing symptoms of herpes, attributing them to something else or just not having symptoms of herpes

Terri
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