First rule, never show anger or frustration. Always remain calm and confident. You can be firm, but not angry or animated in any way. Negative energy from you will only feed his aggression.
Here is an article from Victoria Stilwell about resource guarding:
https://positively.com/dog-behavior/...urce-guarding/
Is your boy generally a happy dog or more serious overall? Does he get enough exercise? A daily routine also helps with obedience and confidence.
No teasing either. Definitely a no-no with dogs with guarding issues.
Max guards chew bones. I hold the chew and him to prevent choking. He tries to get away from me and will growl aggressively. He has amazing bite inhibition though, and I will just remind him to settle down.
Withdrawing attention is also very effective. When Max is naughty and won't listen, I say calmly but firmly, "Okay, I'm leaving the room." I cross the baby gate where he can't follow. That always ends the unwanted behavior.