I think it needs to be tailored depending on your ds's grandparents values, how often they see your ds and whether they actually need to know and why (it may be that they don't need to know, just yet).
My Dad comes 3 times a week, gets my two girls fed & ready for school in the mornings when I go off to work (shift work). He does this 2 or 3 times a week.
My dd2 is under the developmental Paeds. Her behaviour is not easy. My Dad describes her as a 'child who needs to be managed' and he has his own way of 'managing' her. She is often difficult, rude (e.g. yesterday, which dd1 told me about) and has been known to have a meltdown and refuse to get in the car on his watch. She goes on sleepovers to my Dad's but he won't take dd2 without dd1.
He is totally accepting of dd2 whatever life brings and gets on with it however she behaves. I only hear about the more extreme examples of her behaviour, other bits dd1 generally tells me about.
It doesn't matter to my Dad whether she has a dx or not. He knows that the process is underway for dd2 but although he is on the receiving end of her behaviour does not really understand the criteria on which she will or won't be diagnosed. It's not something he is bothered about tbh.
He knows about the referral and diagnosis process because he is there to support me in it and with all the stress and uncertainty (and constant liaison with school) that it brings. That is the basis on which we've told him.
Good luck with whatever you decide.