If you have a dog that threatens other dogs or people when they get near his dinner or treats, there are steps you can take. However, if the problem is so serious that you are afraid of the dog, you need to consult with a behaviorist (only one who uses positive reinforcement), who will work with you.
First, you are going to teach him to "take it". After he's good at that, you will teach him to "leave it".
By working through these steps, he will learn the words Leave It, and what it means.
(Thank you to Karen Clouston for these directions on teaching "leave it")
Work on "object exchanges." Many dogs have been taught by humans to guard anything in their mouth because as puppies they were chased and had their treasures removed.
Recondition the dog to feel relaxed, rather than aggressive, about giving up a toy by offering to exchange with the dog for something better. Start with a toy the dog feels lukewarm about. Show him a treat, and when he drops the toy to take it, say "yes!" and give him the treat. Once he’s doing this consistently, add the cue "drop it." Then try it with a more popular toy. Once this is going very well, touch the toy with your hand and say "drop it," reinforcing heavily with food.
Repeat the above exercise using food. Offer a large dog cookie and then show him the piece of broiled chicken in your hand. Tell him to "drop it" and exchange the chicken for the cookie. Then also give him the cookie ... double reward for giving up food! Work up (slowly!!) to exchanging a marrow bone or other prized food for some delicious human food.
Teach your dog cooperation by playing the "two toy game". Use two (or three) of the exact same toy, then throw one toy. Your dog will run out for it, and as the dog returns, show him a second one. Toss it in the air and catch it yourself or slap it on the floor to make it seem more attractive than the one he's got in his mouth. When he drops the first toy, throw the second, then pick up the first and repeat the whole thing. He’ll quickly figure out that dropping the toy makes you play, while keeping it makes you quit.
Work on "food bowl games". You can recondition the dog to think that hands near bowls are good things, while never provoking a bite or a growl. After you put your dog's bowl down, add a few really tasty treats (different from her regular food) as she eats. After a few days of this, keep your hand in the bowl for a few seconds while the dog eats the treats (not his entire dinner). Occasionally (once a week or less) pick up the food bowl, add the treats, and immediately put it back down. The dog should soon look happy and expectant when you approach her bowl, not distressed. If she appears nervous, you need to back up to step one. Add treats occasionally throughout the dog’s life to keep her response to your approaching her bowl one of anticipation of good things to come.
In addition to this, make sure your are not over-greeting the dogs. Ignore everyone when you enter the room, and you decide when and who you will pet. Everyone should sit for pats on the head.
It's ok for the dogs to be in the kitchen, but everyone should be sitting or lying down. No one should be crowding you at the counter. If they do, simply walk into them gently, and move them away with your hip. No eye contact, no words, just push them (or herd them) out of your way.
Also, have Roscoe drag a leash in the house. Use it when you need it. If you need to keep an eye on him but you're busy, tie the leash to your waist. It's a form of soft control.
Basically, if you do a little day-to-day obedience with all the dogs (while food is prepared, etc.), and make sure everyone has to comply, everyone will decide you're boss and no one will try to assume that position for themselves. I would think part of Roscoe's food issues is just him trying to decide how he fits into the pack, and food in just a convenient excuse (or perhaps a habitual one) for pushing his weight around.