This isn't one of the three dragons of legend. The implications of that are potentially terrifying. But most dragons can understand human speech and respond, so it's worth a shot.

He has his wand at the hip just in case, preparing a quick defense spell in his head, but he'll try keeping calm first. This dragon looks somewhat unsure of itself, like it's sick or something.

"Are you all right? I'm not here to hurt you if you aren't here to hurt me."