Ok so my fiancé won't tell his parents about us

They know we date and live together but they don't know we're getting married in 3 months. We've been engaged for 8 freaking months and the way my mother in law found out was when she saw the ring on my finger months after he gate it to me and she asked who gave it to me. So I said, your son. We aren't making wedding ceremony right now or anything, but we are getting married and I think they should know. I never complained about it to him, yesterday it was the first time I did it. I respect him and the way he is with his parents (closed person) but everything has a limit. I'm sick of this. Is he ashamed or somethin? I don't get!!