We adopted a 2 year old female Chow 4 months ago and we have learned how to make her calm, come (very food oriented) no chewing and combing, which she is still uncomfortable just seeing the brush. What we can’t seem to help her with is when our grown son visits once or twice a month, she hides under the wood stove and growls. We usually tell her no, squirt with water, or ignore. Her first visit with him the first week was great. Her second visit she was laying near him, belly exposed as he was petting her, he put his face near her to give her a nuzzle and she bit him on the cheek. Of course we yelled at her, put her in her crate. So maybe she associates him with that time. He so sad he can’t connect with her. She has not done this with anyone else who visits.
I’m can’t be sure what’s going on here but I can say when Chow’s were at the height of their popularity years ago this was pretty common. Veterinarian friends of mine were always worried when they came into the clinics for exams as they would lash out without warning and do some pretty serious biting rather then inhibited warning type bites.
Are you confident that she truly is only this way with your son or have you been taking precautions around others since the incident? If it’s the former there may be something in her history that she connects with some aspect of him that is triggering a fear. If he were to be around more there are things you can do that would likely override the fear but it’s a lot harder when it’s periodic visiting.
If she’s as food motivated as you say try leaving a leash on her all the time your son is around so she can’t run away (don’t force a meeting – just interrupt the fleeing) and test to see how far away he must be when he tosses a treat before she’ll eat it. Work on reducing that distance very gradually. (very gradually)
For what it’s worth putting your face near a dog you don’t know really, really well, particularly a rescue, and certainly a chow is pretty risky.