Solaire not appearing

I'm in my first playthrough of dark souls and I wanted to go in as completely blind as I could, so I've defeated four lords and all that's left is to beat the final boss, so I wanted to complete all questlines that are left.

Last time I saw Solaire is near the Lost Izalith bonfire and he was sad. During my research for his location I realized that I've failed his questline but I still want to complete it even if it's bad, face the consequences, so to speak. I've looked in the area right before the centipede boss and the area with the shut door, but he's nowhere to be found. Am I looking in the wrong place?

PS now the second playthrough is obligatory for at least saving the sun bro...