Sorry, Mike, but "Seth" most likely won't do as an SCA name. Although the name dates back to biblical times, it only came into common usage beginning in the 18th century, when Old Testament names were desirable among Protestants seeking to break away from the New Testament names most often associated with the Catholics.
If your son is determined to use this as his SCA name, regardless, he'd most likely have to alter it to "Shet", or "Sheth" (pronounced "Shayth") which is the original Hebrew form of this name.
-- Nanaea