I would'nt panic too much, he is just trying to stamp his authority. Have you had him castrated? An entire dog is likely to be far more dominent and less willing to please than one that has been castrated. Have you taught him to leave? I would be doing lots of work that put's you in control, for example: practice sit's, waits, recalls but especially practice leave, if you tell him to leave and he then allows you to take it (slippers, toys, or even his dinner) you then reward with a treat. He will still get the praise and so you make it worth his while in doing what you ask but he is living by your rules and is doing as you ask. The main thing is do not let him think he can intimidate you, if he grumble's at you and you let him keep something you are simply reinforcing that you find that acceptable behaviour. If you have a crate and he shows any unacceptable behaviour I would also put him back in the crate. Simple things like making him sit and you going through a door before him, or sitting for his food, make him work a little bit harder for his praise or treats so he respects you a bit more. Don't give up on him, he is just pushing the boundaries (just like my 5yr old son lol) and personally if he is not castrated I would get him done. If you are in need of guidance you can contact a behaviourist to help you, if you are not sure where to go try APDT (Association of Pet Dog Trainers)
Good luck

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Life is short, live it to the full
2 pets:
Bomber - DDB x Neopolitan Mastiff
Sooty - Cat