Are you asking if he can still rely on the S21 ?
Was it properly served?
You can try to argue that this S21 notice was not properly served. That might well succeed as, if what you say is correct, he cannot show evidence that he did serve it correctly.
However, the reality is that he can always serve a fresh and compliant S21 notice upon you to bring the agreement to an end. The fact that you set up a direct debit does not prevent him doing so if he wishes.
Contesting it at this stage will buy you time but not do much more than that in the long run.
Sorry if that is bad news.
Can I clarify anything for you?