NicholasW comments: I think he would have said that the major premise is part of the logical structure of the world. Being mortal is an essential property of being a man.

And in the long run, it doesn't matter. The validity of a syllogism is not dependent on the truth of its premises. The syllogism:

All lizards have feathers
All robins are lizards
Therefore, all robins have feathers.

is logically valid. The lack of truth of the major and minor premises has no bearing on its validity nor does it make the conclusion untrue.