Absolutely, if he wanted to. But all he knows is the message contents,
where it is going, and the fact he got the message from another
remailer. He will not know the original source of the message. If that
is more than you want to reveal than you need to encrypt to your final
recipient instead of sending a plain text correspondence. Of course,
this isn't always feasible. The final recipient would need to have PGP
on his computer, you would have to exchange public keys or a
conventional password beforehand. It's really up to you the user to
decide just how much security you require for a particular message and
take the necessary precautions.