same don't know why 1)i reset my console = dont work 2) loaded to warlier save = didn't work 3) stood looking at him for ages and guess what didn't work

this happened to me also at my first fight with him i reloaded my previous save and not autosave and it worked perfectly i saved as soon as i collected the last dragon aspect shout

if you killed his dragon or bended the will of it, it dosent work, well for me anyways, because when i reloaded save and didnt kill dragon he absorbed it, and came out of ethereal

TTGAres wrote when ever i try to kill him he uses his wrirlwind sprint absoebs the dragon soul and then he just stays in the middle

Use the Dragon Aspect shout and after he whirlwind sprints it will become easier to take his health down each time. Also the dragon that you rode over to him will try and kill him so use him as an advantage.

This happened like 15 times for me, I found out that it happens if you jump off your dragon, kill both dragons with magic from your own then jump off and finish him, it should work fine after.

1) If you're using weapons created from the restoration glitch then that's what happens
2) You can actually just kill the 3 dragons first and then kill him (much easier)