Moores, I can understand your concerns. ADHD, like everything can be vary from mild to severe. My experience is of mild ADHD/ADD plus dyspraxia and giftedness.
If he has ADHD, whether mild or severe, he will be showing the same behaviour traits at home as he does at school. Your description of his behaviour at home does not fit well with a diagnosis of ADHD. Perhaps what everybody should be looking for is the reasons why his behaviour at school has suddenly deteriorated.
The problem is that ADHD is very fashionable at the moment and too many teachers pin a diagnosis of ADHD on any child, especially boys, whose behaviour in class causes an element of disruption. ADHD does exist but it is not the only reason a child is disruptive at school. Children can be disruptive in class for many reasons, family problems, bullying, boredom, possibly because a child is struggling with one or more aspects of the curriculum and plays up out of frustration. Boredom is a particular problem with very bright children and those with dyslexia. Other possibilities are sight or hearing problems. A visit to the optician and a hearing test are worth considering. Minor hearing and sight problems may not be obvious at home but can cause problems at school.
The best advice I can give you is to press for a referral to a child psycholgist for a full educational assessment. The paediatrician your grandson is seeing should be able to make an approriate referral. Such an assessment will look at him across all aspects of his life and not just at whether he has ADHD or not.
A friend of mine had a son thought to have ADHD. It turned out that he was a very active child but his main problems arose from the fact that he was a very bright boy with dyslexia and most of his problems arose out of frustration and boredom at school because his dyslexia was inhibiting his intellectual progress. With appropriate help he has since done very well.