Nebo's never growled at me over any toys/treats/etc. (he will growl at other dogs if they get near his food however) He'll let me take anything out of his mouth.

What I would do is to let him know that the bone belongs to you, you are just letting him borrow it. Hold the bone while he chews on it. Praise him and pet him while doing this. Gently take it away now and then. If he is good about this, reward him with a yummy treat and praise, then give the bone back. I wouldn't allow him to chew on any bones/etc. by himself for a while.