I must point out the significance of this chapter's title. He is his 'Mother's Son' in far more ways than one.
I wasn't going to elaborate on the above (I like making comments like that and leaving it - I always just expect people to know exactly what I'm getting at without me pointing it out), but I feel like explaining:
Most importantly, it is clear that what happened in his early life (to his mother and father) had a bad effect on him - it left him vulnerable in the same way Joan had been vulnerable when Foul managed to take a hold on her - and clearly this vulnerability was found quickly by Foul. Thus, he is truly his Mother's Son.
Edit - Actually, this has made me want to comment on an observation I have made. I read this chapter, and I see all sorts of things - possibilities, implications - and I can pick up a good idea of what is going on (or at least, it seems to fit because of the earlier Covenant books). Other people seem to read it and don't think there is enough information to guess anything yet. Odd. I think I'm this way with a lot of books - the more so the more I read. Back when I first read the second chrons (ooh, a whole three/four years ago, I think), I wouldn't have picked up things this easily, even with the knowledge of the first chrons I had. It's why I feel sometimes that i'm not getting the full effect of events, revelations, etc in books - I've usually already guessed them.