This is not part of the storyline at the moment, and can be completed later if so wish.

That being said, as posted above one good method is to use a quickening chain near 50% health... get it off before he casts renew because renew goes before quickening chains, like most other annoying spells in this game....

It is not sold in stores yet that I am aware of, but there is also another method that is helpful. Disease will offset his Renew because he will never get below 50% life.. Disease makes the maximum HP you can heal to = to your current HP. However there is only one way I know of Disease being available to you at this point... and that is through a certain accessory and remedies with the appropriate lore.

Thirdly.. he is said to eventually run out of Renews... though some will claim its 5 renews, others 7, others 30+.