Welcome to the Forum finman!
Sorry to say there is no way to get the key unless you re-load a prior saved game and say the word "key" to him. I forget which word actually prompts him to put it in your word list, it might be "Kerah" or "angel". Either way I think you can just type it in when you encounter him there in front of the Serpent Temple.
You won't miss anything but one or two of ankhs and a little more of the story line if you just skip having the key. I didn't get it in my first playthrough either. It won't affect your game either way.
Moni