Dont know if this will help but I found this information on the web:
One of the most recognized syndromes associated with HHV-6 is roseola, also known as exanthem subitum. In this infection, the infant or young child has a few days of high fever, irritability and other symptoms, such as diarrhea and cough. When the fever finally comes down, a rash develops over much of the body, especially in the groin and the neck. Most infants recover uneventfully; however, encephalitis, a severe infection of the brain, is a rare complication of roseola. HHV-6 is also frequently the cause of a syndrome in infants and young children that is even more common than roseola, high fever (without a rash), often resulting in seizures (convulsions).
HHV-6 can also cause encephalitis, leading to coma and brain damage, even in the absence of any other more typical findings of infection, including fever. Studies show that this virus frequently can be found in the spinal fluid of patients with roseola, febrile seizures or encephalitis, including individuals who have no signs or symptoms of brain infection. Often, HHV-6 can be detected in the spinal fluid years after the initial infection, which has caused speculation that a number of neurologic disorders may be due to HHV-6, including many cases of encephalitis and multiple sclerosis.
However, it is very difficult to definitively prove the relationship of HHV-6 to encephalitis in an individual patient, because many people have detectable levels of HHV-6 in their spinal fluid for years without developing encephalitis. While I cannot, of course, discount the possibility that your boyfriend's son had HHV-6 encephalitis, I am not entirely convinced that he did. Of note, herpes simplex virus 1 is a far more common cause of serious encephalitis, and it frequently leaves a patient with severe neurologic problems, even with treatment.
Regardless, you are at virtually no risk of developing encephalitis from contact with your boyfriend. You are very likely immune to HHV-6 already. Even if you were not already immune and you did, by chance, acquire the infection from your boyfriend (HHV-6 is spread through the saliva and respiratory secretions), you would most likely have a mild or even asymptomatic infection. Encephalitis is a very rare complication of HHV-6 infection.