Could be an organic cause. Take him to the vet. and check out if your JR is suffering pain. A blood picture will help and make sure that his physical health is absolutely ok.
Maybe he has made bad experiences with human hands and now has fear of it, so before he has to worry about violence through the human hand again, he lets nobody in to make sure, that everything will be ok and safe for himself.
The muzzle you have chosen is ok, insofar he can drink and hackle with it, that is very important.
Then you have to look for a professional dog trainer, because if this situation remains it will get worse, for you and the dog.
Here I will give you a first aid for problems with your dog to compensate its behaviour. Things will not get worse as fast as it would be without this programme, but without a professional backup it wil by too much to lady luck.
Anti-control programme
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english version will give you further information
In the world of dog education: everyone does what he wants, but nobody does what they should, however everybody plays the same game.