To open a large dogs mouth- you did what you should. There is a spot on the gum above the canine- the skin is soft and if you apply pressure- they will open their mouths. Its like a muzzle pressure point- even used by vets to open the jaw of a dog under anethesia..
Take your hand on top the of the muzzle = with thumb on one canine lip area, pinky finger on the other side- gently press and say drop. The dog will let go- and as soon as the dog does- praise. Even offer another toy instead. ( pig ears withstanding but something else would work as well).
This is also the same part of the muzzle a dominant dog would mouth a subdominant dog. Do not apply much pressure- be sensitive when it opens. Attaching a brief video using hottie as a model to show you what I mean. View My Video