Who was it said: "You should be a grandparent first, then a parent"?
This thread is so sad. There's plainly a communication gulf between child and grandparent.
But the onus has to be on the grandparents to build bridges.
Perhaps the OP could suggest some things the grandparents could do that a kid might appreciate?
One thing that springs to mind is: A subscription to his favourite comic, so that he has something to look forward to when he visits and something to occupy him while he's there?
Do they have a local shop they could take him to, to pick up groceries (but secretly to buy him sweets)? This would help distract an energetic child.
Pocket money: He may already be getting pocket money from OP but a top-up from gran and granddad is always welcome, however small.
Food! Kids are always hungry. Organise with the grandparents to eat together at the table, and then make sure he turns up hungry (or at least peckish, if 'hungry' would lead to a meltdown!).
Explain to the grandparents that they get the enjoyable end of parenting, with the bonus that the child goes home after a bit! So they should make the most of it.
I speak as someone who had a hugely enriching grandparental relationship while I was growing up. My paternal grandmother was like a second mother to me. And yes, they spoilt me rotten. But that indulgence set me up for a lifetime bond with them. I have a picture of them on my bedroom mirror (with my granddad proudly wearing the scarf I knitted for him for Christmas when I was ten) and there isn't a day goes by without me looking fondly at it, despite the fact that my granddad died in 2001 and gran died in 2010.
OP, I hope it's not too late to salvage a decent relationship between the generations. Good luck.