I'll try and make this short. I assume that everyone is using their best stuff in this battle so know that I am too. After being hit several times with this glitch I found my answer online. Its not the soul, its his health. Try and keep his health no less than 1/4 (or on screen 1 inch of red) showing. The A.I triggers then that he needs to soul recharge. If his health doesn't fully recharge, he won't move. I'm an archer so I hit him twice with arrows, then used a regular dagger to get him the rest off the way. Now here's the trick: on the last time (the last dragon soul) he took way more damage with my bow than the first two. So I shot him once then used the dagger. As long as his health fully recharges, he will move. I was hit with this glitch last night, and beat it last night. I can go into more details if anyone needs me to, just ask. Sorry for the length, hope it helps.
I received an update from Bethesda Support today in regards to my query:
"We're definitely aware of the glitch, and we haven't denied the issue at all. The issue that you are experiencing is a known issue and we are working diligently to fix it in upcoming patches. Unfortunately at this time we do not have an ETA on when the next patch will be deployed. Aside from loading a previous save file prior to reaching that point in the quest and trying again (which we understand doesn't always work in the case of this particular bug), the best thing to do at this point would be to keep an eye on our forums, as this is the best source of information for emerging issues and patch information."