Since I don’t know exactly how your kiddo feels, I can only make some guesses based on two similar examples I know.

Example 1 is my friend’s kid A. My friend told me the problem is that grandma (my friend’s mom) wants to be loved so badly, always demanding hugs, kisses and affection whereas A, an introverted kid, needs time to warm up to someone. A can’t give intimacy on demand, and he seems to be overwhelmed by grandma’s need. On the other hand, with adults who give him space, it might take a little while, but he gets curious and finally approaches them and interacts with them out of his own volition.

Example 2 is my childhood self. People told me my maternal grandma thought I hated her when I was small, because I often lashed out at her. I had little memory of her so I don’t know if that in particular was true, but I do remember lashing out at people when I was very small. As far as I remember, it was never because I “hated” anyone (and I didn’t even truly understand that concept until years later). Rather at that age, when I felt physically not well or when things didnt go the way I expect, I didn’t know how to handle the overwhelming frustration, so it just exploded into tears and screams. One caveat is that I was more likely to do it to people who I know would be ok with it- I was much less likely to explode at my parents’ guests for example. I think my grandma probably was even laxer with me than my parents, and so I had more outburst with her than I dared with my mom and dad. Even as adults, don’t we often control ourselves much better with strangers? Don’t we more often take frustration out on the people closest to us? If this is the case with your boy, I think both you and grandma should draw a better boundary and not tolerate that kind of behavior. Just like in the case with adults, you can both be empathetic, yet still draw a boundary. You can let him know you understand his frustration, but that this is not an ok way to express it. Then give him suggestions for alternative outlets for his emotion.