Maybe something with rollers? That worked for one of my horses!
I agree with Lucy Balmer! Try a variety of gentle bits... don't use anything harsh that could make it worse.
A Waterford bit. Horses who like to open their mouths usually like to take the bit and play with it. A Waterford has so many links that it just bends when they try to play with it so they can't clinch. Hope that helps!
If his mouth is open, he's showing you he is in great discomfort. I advise you don't buy a flash noseband as this does not solve the problem and hides the pain he is showing.
All I can advise is that you try a variety of different bits. All horses are suited to different ones. The bit you use depends greatly on your horse's mouth anatomy.