O.K. I am going to be the dissenting opinion here. While I agree outside help seems necessary ASAP, I don't agree with all of the advice at the link.
@basilboy7:
I told my vet this and she advised me to close his mouth until he calms down which I find works.
Sometimes you have to go with what works, particularly when you haven't been able to get the correct response the "right" way. Bottom line, you need to get control of the situation and stop allowing the behavior to become a habit.
In practice, most people are not going to be diligent enough to maintain the soft mouth Dr. Dunbar talks about, so IMHO it is better to simply teach "no mouthing" right from the get go. And BTW, Dr. Dunbar does not rule out physical restraint. Specifically, he says, "It is much better for you to walk away from the pup than to physically restrain him…."
I would suggest that some methods do not work with some dogs, and if you are not getting the response you hoped for it is time to try a different approach. Physical restraint may not be ideal but does the trick with some dogs. I know people who teach their pups not to mouth in exactly that way (holding the mouth closed). I've used it myself in some cases and it did not make things worse for me. I do think it makes it clear to the dog what you are objecting to. What you don't want to do is make it into a game where the dog nips and withdraws so you can't hold his mouth closed.
As regards the "yelping" only increasing the behavior, I think we may have inadvertently caused confusion with squeaky toys. By teaching our dogs to mouth things and be rewarded with squeaks, we may be training them to bite hard enough to get an entertaining sound as a reward!