My boyfriend suspects me because of the words I typed in the input method. How can I explain to him to make him believe me?

Since he doubts your typing input method, let him see your mobile phone typing face to face, and what he sees with his own eyes will naturally dispel his concerns.

If I doubt you again, it means that such a person is too stingy and suspicious, and I should break up with him at once. Even if you marry him, he will suspect you in the future. There is no need to spend a lifetime with a suspicious person!