Here is a great online article that will really help you to understand your dog's resource guarding and how to help him.
RESOURCE GUARDING – HOW TO TEACH YOUR DOG TO SHARE by GRISHA STEWART Resource Guarding â€“ How to Teach Your Dog to Share | Grisha Stewart
A bit from her article:
Here are a few of the myths
about resource guarding, according to Jean Donaldson's book "Mine! A Guide to Resource Guarding in Dogs."
Myth #1: Resource guarding is abnormal behavior.
Myth #2: Because resource guarding is driven largely by genetics, it can’t be changed.
Myth #3: Resource guarding can be cured by making a dog realize that resources are abundant.
Myth #4: Resource guarding is a symptom of “dominance” or “pushiness.”
Myth #5: Resource guarding is the result of “spoiling” a dog.
So if the answer is not to "dominate" your dog or shower it with freely available food, then what is it? Simple. Make your puppy or dog understand that the approach of a human to his food, toys, space, etc. is a Good Thing.
The process is called classical conditioning. Just as the clicker is associated with treats in your dog's mind, the approach of a human hand, face, or other body part to his food dish should mean better food is on it's way.