What ginagt says is a good idea, try lots of positive reinforcement. If he growls it's even more important to get him to drop it and reinforce the praise for him dropping it. If you allow him to get away with growling it may be a much larger problem down the line. When we got Bear he was 5-6 yrs old. We don't know his history other than he was severely neglected. He was also extremely food and bone/rawhide aggressive. We could take anything else from his mouth without a problem, and other than that he was extremely laid back. He even went after one of my other dogs for just walking by while he was chewing a rawhide and he's not an aggressive dog. We're now at the point that we can take his food bowl without a problem. The rawhide we still can't remove from his mouth, but we can get him to drop it and walk off if we tell him. We did a lot of positive reinforcement, everytime he got a bone, we'd make him drop it and reward him. If he growled he was told no, then to drop it and if he did he'd get a treat. It took a lot of time and patience, and we still have to reinforce it.