My boyfriend says he feels a ring like object (cervix) around the head of his penis, what does it mean when the cervix is open?

Quite normal. For him to feel cervix with intercourse. The more you have vaginal delivery or labor the more likely your cervix will be wide at the opening. Close-open cervix is a clinic term used by ob/gyn when you are pregnant, on labor, or at risk of miscarriage. Hard to explain in this format. If you are not pregnant, your cervix is open. If pregnant it should be close, until you go on labor.
Normally not open. A normal cervix is not that open. About the only time a cervix could or should be that open is for a few days after a normal vaginal delivery. If your cervix is open enough to allow penetration of a penis, then it is time for you to call your doctor.