The main character really needs to start trying to talk using words. He possesses level 3 skill in the dominant human language. He has a mouth, teeth, tongue, and lungs to expel air. Literally experiment to make sounds. It makes no sense that he shouldn't be able to do this in DRAGON form. But the author pretends like it's only something he can do while being a human. There is nothing preventing him from trying, other than the author clearly want's communication issues to be a main storyline and humor source. But they provide no justification for it, like a curse that would make him silent, or something.