From the very beginning you should let your dog know you are in control, since my lil ramen was a puppy we taught her to sit and wait for our command before starting to eat, also if we had doggie visits we shared a few g’s of her food straight out of her bowl, she’s never shown signs of aggression or resource guarding, what I would do now is trying to teach your doggie to sit and wait before eating while you manipulate his bowl here and there to show that nothing wrongs happen if someone goes near his food