If he says the truth, all Cretans are liars, and so he is a liar and he is not saying the truth. Okay, so this is not possible.
If he is telling a lie, all that we know is that not all Cretans are liars, we can't imply that all Cretans are truth-speakers.
A week later, he says that all Cretans are liars, which he have already discovered that isn't true, because he lied, and contradicts him. Because, in the previous week we proved that he was saying lies, so he is a liar, therefore in the second sentence he is lying too because he lies by saying that he is not a liar.