It's important that he does well in school and has a good life and I hope that works out for him. It's a myth that sugar causes adhd, it's also a myth that if you give a kid with adhd sugar that they will get hyper just because they have adhd.
The thing is giving any kid a healthy diet is good, cutting out sugar completely is not nessassary though, unless a doctor has said so because of a medical condition. I hope you and the family can come to an agreement on what he should eat, but really, everyone in the family should be eating the same way (unless diabetic, lactose intolerant, etc.). All kids, adults even, get an energy boost from sugar, but it's not an uncontrollable thing and it doesn't change a person's attention span either. Since your grandson is not diagnosed and therefore not being treated, my best advice to you is (for the whole family) eat healthy, some sweets are fine in moderation, eat vegies and fruits too, and get excercise. Kids are going to be hyper if they don't have time to just go run around and be wild, they can do that productively though, like join a sports team, or just go to the park with them, etc. Unless his school has said he has bad behavior and thinks theres a big problem, or you've seen behaviors in him that put him in danger to himself or others, I don't think you have much to worry about. I realize you're a nurse and I'm not trying to tell you what to do or say I'm sure of this because really I can never be sure of a situation I don't know much about, but there's my two cents worth. good luck