hello!
Glad to see that you had the common sense to have his teeth checked as it could have been pain that contributed to it. Do you have regular lessons? Can your teacher ride him with his mouth closed? It could be that he just goes with his mouth open and that is the way he is, or it could be that with a bit of schooling that he can be trained to close his mouth. if he is still accepting the contact and rounding niecely, it is probably not too much of a problem.
If he is hollowing and holding his head in the air with his mouth open, it could be a sign of pain somewhere, have you had his tack fitted recently, for example.
I hope you can either sort it or learn to life with it! Good luck! Do let us know how you get on.
bye!