Grandparents spoil kids (it's their job), when the 8 year old is having fun with the great grand parents, I would think the last thing on his young mind is to call mom and dad.
And although he seems totally capable, I think the 'kids will be kids', kinda creeps in. I am a whimp dad in some areas, I think life came too fast for me in some aspects. So I try and cut my kids some slack, but discipline remains the same.
You no doubt trust your grandparents, other than worrying about their well being at their age, just teach your son how to call 911 (God forbid) should a crisis arrive.
Just have him carry his cell phone with him when ever they leave, they have endless gadgets to attatch them to the little person so they can't loose it (well that will always be the dream)
All my grandparents died before I was born, so I don't know what I am missing, but I would say let him enjoy
