Board Thread:Lore Discussion/@comment-6702118-20140722142914/@comment-5989209-20140723063509

Dragon's don't have mortal bodies. They only die when their souls are ripped out. Considering we didn't absorb his soul, you can't say that you killed him. Without being in Shor's realm, where Akatosh also has influence, being of the same soul as Shor/Lorkhan, Alduin was still unable to be killed. As for why he is weakened, no one knows, but their are a few suggestions that involve him not consuming and such.