Pretty sure files aren't encrypted on Dropbox's servers, maybe encrypted on the journy.
I'd gpg -c the wallet.dat and use a strong password before trusting other people with it.
He said, "And I want to back them up on my DropBox account. That i encrypt with Truecrypt."
He has a Truecrypt volume on Dropbox. So HIS DropBox is encrypted (no one else's is, technically).