Answer:
Charlie will start losing his ability to write his thoughts.
Step-by-step explanation:
What the final line of this excerpt foreshadows is that "Charlie will start losing his ability to write his thoughts".
Charlie pointed out the need for him to be careful in the way he writes. This is to enable him not to loose his ability to write and communicate his thoughts clearly and simply with his audience.
So, the last line foreshadows that if Charlie isn't careful, he will lose his ability to write his thoughts.