Note to the developers: Great job guys. The latest XBOX 360 patch has made it impossible to kill Alduin. This makes it somewhat harder to survive his attack as he never looses energy. Not even Parthenax's attacks hurt him. My saves are useless because they don't work without the patch which basically means I get to start all over. I almost snapped my disc in half when this happened. If this keeps up it will kill my enthusiasm for the Elder Scrolls franchise. Having completed most of the other quests, this really upset me. Way to drop the ball...

Did you smack him with Dragonrend?

During the quest, Alduin's Bane, there is a bug where Alduin becomes invincible if you fail to defeat him after several tries (in other words: you die and the game reloads from the autosave several times). This problem was around before the latest patch (1.8) came out. The only solution that I'm aware of is to quit the game (exit to the xbox dashboard) and restart it, then reload the save. I've experienced this problem myself several times and this solution works.

this
this bug happens from time to time, with the latest patch or not
also happens with a few other creatures from time to time like the dremoras a dagons shrine and the wolf spirits in the companions questlines
best to just reload from a save before the encounter initiates at all
also, he cannot be harmed in the air at all, he has to be downed with dragonrend if that helps any
best to not bash a patch about a bug you think it has created without checking around first
this bug existed long before the past few patches even