It is believed that along with genetic factors, herpes infection with HSV-1 and HSV-2, is one possible cause for MS.
Actually, it is believed that herpes viruses are either cofactors in its etiology or triggers of relapses.

You should take antiviral drugs as preventive treatment to reduce the frequency of genital herpes outbreaks, use protection during sexual intercourse and avoid sexual contact when the infection is symptomatic.

Your boyfriend can consult his neurologist if he can benefit from antiviral therapy, too!