Best thing to do is get in the habit of modeling things with the mouth open, and get in the habit of doing it, same way for human characters you model, leave the mouth at least slighly open and the fingers on the hands spread out . . . it will save you a lot of greif.
At the point you are at though, you could do one of two things, paint weights . . . find a tutorial for that somewhere on the web, or, un-skin, re-model it with the mouth open, re-skin. Unskinning is done near the same menu you skin it with.
I have also found adding extra joints and manipulating them can save you a lot of time once you get a "feel" for how they are binding to and effecting the skin.
"The Sage as an Astronomer: If you still see the stars as something above you, you lack the eye of knowledge." Friedrich Nietzsche