it may just be "too much bit" in his mouth, some horses have a thicker tounge than others and therefore less space for the bits. He might just get used to it after a while or you could try a thinner curb bit, yes thinner bits are considered more severe (and they are if used incorrectly, as any other bit) but it might still be more comfortable for your horse. It might also help to try another shape of curb bit. But finding the right bridle and bit combination for a horse is tricky especially with double bridles.
Thanks @davidlucasrider. Can you tell me how I make sure the bits fit him? I have kept my contact very light as my horse is light in the mouth as well. He has had a good neck posture and done all exercises well with the double bridle, even the forwards and downwards neck on the trot, so I can see he is seeking the contact. He has also jept above the vertical. The only problem seems to be the open mouth.