I think that askng for his blessing would be the way to go. As poptart said "permission'' seems too old fashion like he still makes all the decisions in her life. I do know some couples feel asking the parents is very outdated but I''ve always thought it was a sweet old fashioned touch.
My exFI spoke to my father and mother but he simply told them that he was planning on proposing to me over such & such a weekend and hoped he had their blessing. Of course both my parents were very excited and happy to know that we''d finally be engaged but then my exFI also knew how close I am with my family so he felt is was okay to talk to them.