After I use dragonrend to make Alduin land and attack him a few times he becomes unkillable, and I can move through him...my attacks go through him and he's permanantly stuck on his side and doesn't move....at all.

During this quest, if a player attacks Alduin before the initial speech to the player is completed,

a bug may occur in which the player can no longer deal damage to Alduin, regardless of attacks.

• Reload a save before the dialogue begins

If you attack Alduin before his initial dialogue and Parthurnaax tells you to use Dragonrend, you will deal no damage and enter into a never ending battle,

unless you die. Sometimes the bug can cause Parthurnaax to permanently land as well, which is annoying.

• reload from before parthurnaax tells you to Dragonrend

-------- OR

• If you do not have a close save nearby, just run down to high hrothgar enter-exit, and he will be vulnerable again

I'm well past that point, Mark, I'm talking about fighting him, alogside the three heroes, in Sovngarde. But I found the culprit, it was a mod I'm using that's related to shouts. I just simply disabled it long enough to defeat him then I renabled it. Everything went smoothly after disabling that mod.