I gave him the Eye and he gave me the magnifying glass. But he says that his faith is not restored. I assume that I must restore his faith to get him to go to the big head, but how do I do it?

If you can't get him to go to the "big head", bring the "big head" to him - sort of. Talk to Eras; he wants followers/worshippers to spread his word (think bible). You'll find a blank book on Bartalan's island.