dogs of mine that guard bones do not get bones loose in the house. I do trade training because sometimes they find something they will guard and I need it back from them and don't want to get bit getting it. I use something delicious and small treats, such as cheese or hotdog chunks or something he doesn't get any other time. Then I trade for what I need them to leave alone. My dogs are allowed bones only in crate, but reaching in can be dangerous as you described on dogs that are not trained that they do not have to guard their posessions. It is not uncommon and on dogs of mine that are super defensive of the bones, I do not allow bones until I do the extensive desentization training. If you force this issue before he gets the following training he will bite.
So, first take away bone when he isn't watching and put it away for now. Then start training him in and out of crate command using food. Say "kennel" or something like that and toss (small piece) liverworst or hotdog or cheese chunk into crate. Say, "lets go" or something like that and toss treat on floor in front of crate inviting him to exit kennel on command. Start this exercise on leash so you can guide him back and forth without touching the dog or collar.. give command, toss treat in crate, praise when he goes in. Give command for exit "I use lets go".. toss treat on floor (do not feed by hand). Praise when he picks it up. In and out, praise and treat. Then go outside on a leash for a walk...
When you leave, give rescue remedy. Then toss treat on floor, (say get it)... then (off leash) guide him to crate and toss treat in and say "kennel". (reaching in crate to take off leash can earn you a bite) so that is why I want you to do the final entry into crate off leash, take leash off outside crate, dog enters willingly. You can try back and forth off leash too, in and out, so he doesn't think he always gets the door shut ffinally every time you toss the treat in the kennel. Do the in and out training when you don't really have to be somewhere on time.
Again, watch him via video and see what he does without the bone in the crate.
You can try giving him the bone later when he is well established and not challenging you any more.
I have had dogs well trained and lovely temperaments that just cannot handle bones and especially rawhide and if I do the training and not satisfied I have selected a more boring bone for those dogs in the crate, such as a nylabone but with peanutbutter smeared on the outside that they lick off quickly. Really good soup bones or rawhide can unglue some dogs into defense mode. Doesn't mean it is a bad dog, just means you have too exciting of a bone and need to phase back, retrain and start again. Kind of like when I trained in classes and I was using cheese and my dobie was grabbing my fingers hard for the reward. I trained using kibble. He loved it, but it didn't make him crazy and grabby.
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With the rescue he might not need the bone for comfort. Lets try taking away bone... using specific food rewarded command to go in and out of the crate and tell your helper never to reach into the crate for the dog.
Ask him to exit willingly and then let him out for his potty break. I prefer to use a leash for outside walking to get message across if you can. I don't allow dogs to roam free during the first few weeks aclimating in my home for safety sake.
Use "super treats".
Eventually you can do the trade training and teach him to give the bone or allow you to take the bone for a super treat. That training takes time, but worth it.
Let me know if you hit any snag or do not understand what I wrote. Writing fast but needed to get message to you quickly about the bones.
PS be very careful. His growl was a warning, the snap and miss is an escalation of the warning. You are very close to a bite. Need to be very careful not to force the bite. Once he takes a stand reaching toward him will force a bite. Ian dunbar used to say do a "retreating reprimand" where you verbally correct while taking a step back, encourage dog forward using food and reward for forward behavior. I toss food treats into a small bowl on the floor or on the floor itself, I dont feed by hand at first in this training..
I think Ian dunbar might call it an instructive reprimand too.. You could ask for a sit first once he comes forward for the treat. Eventually you can treat by hand and teach him a soft mouth take but for now don't do that..
best wishes, be careful.
