I heard that the Skyward Sword is supposed to transform into the Master Sword after doing certain objectives. Personally I hope this is not true. The Master Sword should be a seperate sword found in the middle of the game, that's the traditional way. Having your original sword transformed into...
Perhaps the shield was passed down by his decendants who lived in Hyrule before. They did that in The Wind Waker. I'm actually kind of positive about this theory.