He is resource guarding, and you are wise to stop it in its tracks every time it happens. Dogs will resource guard nearly anything. Ours will sometimes fight over a certain toy. When it happens, both dogs are corrected (usually a strong word works, but sometimes a tap on the rear is needed to get their attention), and the toy is removed for at least a week.
You will learn to analyze the dogs' body language and know when it's about to start before it actually does. Then you can distract the offender (call him to you and treat and praise). Prevention is the key here.