Strongly disagree with this.
The grandparents love the boy, their worst crime is ignoring him and not engaging with him.
Teaching a child that they must always put their feelings first - unless their wishes cannot be fulfilled, ie unless it’s impossible - is creating a child who thinks they are the most important person in their world. That their feelings come first. This is absolutely NOT the way to create boundaries, which are indeed crucial. It’s the way to create unpleasant and unhelpful characteristics.
Children need to learn that in the real world, which parents prepare them for, life is not going to allow them to prioritise their feelings at all possible times.
Setting boundaries is about understanding that everyone (yes, including them) has limits which cannot be crossed. Mostly society agrees on the main ones - there are laws. Then, as parents, we teach our children that certain things are not OK. It’s not OK to pick on a classmate, it hurts their feelings. It’s not OK to steal toys. It’s not okay to speak with your mouth full, to not say please and thank you etc.
Boundaries do NOT mean teaching a child it’s not OK to feel bored and ignored in the house of your grandparents who love you, and with whom you spend an afternoon once every couple of months. What could possibly be wrong about that?
Just because the child has spoken, he must be obeyed?
My grandfather and I didn’t even speak the same language. He always sat in his armchair in the corner, reading the paper and listening the the radio. We literally never spoke. He told my grandma, not long before he died, that of his 10 grandchildren I was his “favourite”. No idea why, she thinks it was because he enjoyed watching me be a child, and because we didn’t speak he didn’t have to know me (haha). It brought him joy to have me around. I remember just sitting around, listening to my mum and grandma, and reading or looking at my grandma’s crockery or whatever - I don’t even remember. It was a house for old
people, nothing for me to do. I guess he also saw the way I interacted with his DD, my DM.
All to say, don’t make a monster of your son. He doesn’t have a god-given gift to be amused or entertained at all times. It’s healthy for children not to be the center of attention (probably most of the time, really).