Oops....

You can save him by: Progressing until you find a boss called "Demon Firesage" (Don't worry about missing him, he's non-optional) Once you find the boss, don't kill him(if you quit the game and reload, you will be just outside his fog gate) Take the path to the right of his fog gate and kill the bugs until you reach a large locked door. There is another, non respwaning bug on the other side of the door that you need to kill, aim the Poison/Toxic mist pyromancy though the door and use it several times until you see it's health bar go down, you'll know it's dead when you get some souls for killing it. And that's how to save him, you can even summon him for the last boss now if you want! (You can also just open the locked door by giving the chaos servant covenant 30 humanity, but the way I described it is cheaper)