None of the Penal Code statutes under which he could be charged are "serious felonies" as defined in PC 1192.7(c). The most probable statute he could be charged with PC 646.9 which is a "wobbler," which means it can be prosecuted either as a misdemeanor or a felony, but not a serious felony. The time he might be required to serve could be increased if he has also violated a restraining order. Even if he has both stalked and violated a restraining order his outside exposure is 4 years in state prison (of which he'd serve half) and no strike or serious felony label.
If he's been sent to Atascadero he may well not be incompent to stand trial or may be mentally impaired enough that his punishment is reduced.