After reading the elder scroll and the fight with alduin starts I'm stuck. No matter what I do Alduins healthbar is at max. And yes I do use dragonrend and he is till invulnerable after landing. So how can he be defeated ?

Try loading an earlier save or autosave.

Had the same problem today. The autosave from when you get back from learning Dragonrend didn't work for me, had to go back an earlier save.

I don't quite understand. My problem is that Alduins healthbar remains at 100%. How can loading an earlier save and then reading the scroll again help

The glitch happened when you returned from learning Dragonrend. Loading a saved game previous to when you read the scroll will make it possible to NOT get the glitch the next time.

That happened to me, too. My mage was using her dual cast master Lightning Spell and Alduin stayed at 100% health, even with Paarthy's attacks hitting him. I had to reload a save from before reading the scroll to get past that. Annoying, I know, but that's Skyrim for ya

this. the scripting to make alduin vulnerable is broken. you've got to load a save from before exiting the past vision. I like to make a save before reading the scroll because sometimes alduin doesn't land in the vision.