dying does not refill his health, however he does heal if you go in and out. so if you just "check on him", you are screwing everybody else over. check the twitters for health updates.
I think the urdragon got stronger during the grace period(the 15 or so minutes in which you can kill him super easily) with the last patch. so it's not as easy to just walk in there and get all the hearts with a few hits.