Soundsl like your dog was guarding his food bowl from you/your son approaching. Doesn't matter if he's never done it before, it can develop at any time. I would crate him or close him off into an area while he eats ALONE, and play trading games where you/toddler walks near the room he's in while eating and you throw bits of hotdog at him so that he learns that people coming near him when he's eating = good, not bad.
Danielle, owner of...
Fraggle Rock Collars
Fraggle Vom Richcreek CGC CA
Red Dog's Reconnaissance Man CGC
General Patton Vom Winter Storm