Please, please, please do not punish your dog for growling. It's likely why you're seeing snapping. It's also what could lead to an E.R. visit, much more quickly than you would heeding his growls and giving him alternative behaviors. Punishing his growling, his only means of communicating his uncomfortability, fear, anxiety or other emotions, will only create even more of a negative association with you, the crate, and anything else he's experiencing at the time.
This is not his way of challenging you. Dogs do not challenge humans, do not 'dominate' them, or try to rule the household in any way. Any behavior you're seeing from your new dog, is not a result of a lack of respect. It is the human's job, when bringing a dog into their home, to teach the dog what is ok and what is not ok. They otherwise have no clue what is expected of them, and every behavior they exhibit is just a normal, natural dog behavior that no one has taught the dog not to do yet.
He's simply showing you with his language, that he does not want to do what you are trying to do to him, which is put him in his crate. Per his past, he's likely created many negative associations with things that may not seem obvious as first. I'm glad to hear you are aware that hitting a dog achieves nothing but abuse in the name of "training", and are not willing to do that. It does not teach the dog anything, except fear of humans and anything that was associated with the incident, and I'm disheartened that such a young puppy was hit, no matter the size.
However, punishing growling is not productive either. To get him to listen to you in general, you need to be engaging, and rewarding his attention to you. Once you begin to engage with him, and teach him how to give you attention, getting him to move for you or go in his crate should be easier. If he refuses to go to his crate, do not physically force him. Instead, use something he finds rewarding, to call him to you. Throughout the day, reward his coming to you, his attention to you, and his good behavior. If he growls, he is uncomfortable, and using something he likes to get him moving, is much more productive than punishing his growls. Again, you won't change his emotional state, and simply create more of a negative association, and in a dog of this [eventual] size, that can be dangerous.
Again again lol, he is not trying to rule the household, in any way. Any behaviors he's exhibiting are being reinforced in some way, so alternative behaviors need to be reinforced instead. And there is no need to fear his growling, he's simply communicating. Use positive reinforcement to teach him what you want him to do without making him uncomfortable, will help his behavior, as well as your bond with him. If you haven't had him long, as in under at least a month, then he's also likely very confused, frustrated, and possibly a little grumpy. Dogs don't tend to show their "true colors" for awhile after arriving.
Here are some informative articles for you: Understanding Dog Growling and Dog Language - Whole Dog Journal Article Thank your dog for growling - Orlando Dog Training and Behavior | Examiner.com Respect the Growl… Â« Success Just Clicks "How can I correct the growl?" Â« Success Just Clicks Three Ways to Confuse a New Dog
Are you familiar with clicker training? You can also use a marker word. Here are some videos on how to begin training, eye contact/attention, crate training games (so he always
loves his crate), and calming signals and the positive interrupter: https://www.youtube.com/watch?v=eiMGJBxRtBw https://www.youtube.com/watch?v=_wv1uvvqaSw https://www.youtube.com/watch?v=MgnLgHFRJu4 https://www.youtube.com/watch?v=PRT6r6d79OU https://www.youtube.com/watch?v=dUzF0g0PwY4 https://www.youtube.com/watch?v=dUzF0g0PwY4 https://www.youtube.com/watch?v=TBvPaqMZyo8
Stickies to read: https://www.dogforum.com/training-beh...nce-dogs-4076/
(ETA: Didn't see this one, it covers all of what I just said lol.) https://www.dogforum.com/training-beh...rowling-86338/
ETA: I think it's possible that the reason he is refusing to go back into his crate has something to do with being able to be outside of his crate more often. He may have found that more rewarding, and doesn't necessarily want to go back in it at certain times. Here's the crate training thread, as you may want to go a few steps back in his crate training and make his crate more rewarding for him: https://www.dogforum.com/training-beh...ning-faq-3974/
Also, feel free to start another thread on the subject of he and the existing dog getting along. There are some great resources out there about how to introduce dogs, and I can't find a link ATM. Just go slowwww. Do not force them to constantly interact, and give your existing dog time to himself, as well as time with you. Oh, and we'd love to see pics of your pup! I love me some Mastiffs.