What is it called when people guess words?

I don't if there's anything wrong with him, but my cousin keeps guessing words. He's a teen and every time he looks at a word, he guesses what it is. For example, the word can be come and he replaces it with cure. He does it all the time and never realizes his mistakes. He takes the first letter of any word and replaces the word with a word that starts with the same letter. Is there something wrong?