How does Charlie’s use of language decline in terms of phonetics, vocabulary, punctuation, and sentence structure?

During the time when Charlie's intelligence is at its peak, his grammar and punctuation are flawless, as is his spelling, and he writes complex sentences using an advanced vocabulary. Charlie notes that

Dr. Strauss continually reminds me of the need to speak and write simply so that people will be able to understand me.

An example of Charlie's new writing ability is the following:

Dr. Strauss on the other hand might be called a genius, although I feel that his areas of knowledge are too limited. He was educated in the tradition of narrow specialization; the broader aspects of background were neglected far more than necessary—even for a neurosurgeon.

We can see that Charlie has written two complex sentences (sentences with two or more clauses) in a row. The first sentence links a subordinate clause, beginning with although, to the independent clause that begins the sentence, separating the two properly with a comma. He shows a sophisticated grasp of punctuation in the second complex sentence, using a semi-colon to separate the two independent clauses, and using a dash properly to add a phrase. Only a well-educated person would be likely to have such a grasp of punctuation and mechanics.
Charlie uses an advanced vocabulary, such as specialization and neurosurgeon, and his spelling is flawless. He is able to make insightful observations about Dr. Strauss.
As his intelligence begins to fail, however, Charlie begins to write more simplistic sentences, fraught with grammatical and punctuation errors as well as spelling mistakes. For example, Charlie writes:

I get awful headaches and asperin doesnt help me much. Mrs Flynn knows 1m really sick and she feels very sorry for me. Shes a wonderful woman whenever someone is sick.

Charlie has begun to leave out the apostrophes that indicate possessives, such as in shes, which should be she's. He misspells aspirin. He doesn't use a comma to separate the independent clauses in the second sentence. He uses simple, vague words like "awful" and "sick," which don't give specific information about his illness, and he repeats the word sick twice. He is beginning to sound like a child.
By the time he ends his narrative, his words have become very simple, and he is spelling phonetically, meaning according to how the words sound to him and not according to their recognized standard spellings. For example, he writes:

Its easy to make frends if you let pepul laff at you. 1m going to have lots of frends where I go.

While the first sentence above is complex, consisting of an independent clause followed by a dependent clause, it uses simple words that are misspelled, such as friends, people, and laugh. He continues not to use apostrophes when he needs them. He is now writing like a person who is just beginning to learn to read and write. His thoughts are broad and unsophisticated, lacking in nuance.
Charlie's loss of verbal ability is a tragic sign of his loss of intelligence.