What a kind and wonderful grandparent you are. This question is one that plagues parents and grandparents and has for generations. I was started at school at 4 (turned 5 in the March), and it was fine academically, but socially I always felt a bit off and, at the end of Year 12, when I was a mere 16 years old, I really felt it. But that's an anecdote, not data, so what other information is there?
There are a lot of guides, often published by the departments of education in each state and territory, on when your grandson should start school. They say, more than academics, it's the rigour of the school day that should influence the decision, Is he ready to sit still for many hours a day? Can he follow the rules? Can he take turns and wait patiently? Can he work independently when asked but collaborate with others conscientiously and sensibly when asked as well?
These are the kinds of questions you need to be asking. Others involve physical issues like can he wear his uniform? Shoes and socks all day? Use the toilet? These are the kinds of things you need to look out for.
What does his kindy or preschool teacher say? Do they think he's ready for school? What strengths and weaknesses do they identify?
Remember, the prep teacher is there to help your grandson, obviously, but also to meet your needs and help you help him to thrive. They can be an ally in helping him to adjust and make this important transition.
Also think, he's going to be in the middle of the age range, if his birthday was June, you could be forgiven for waiting, but his February birthday means he's a little bit older.
Maturity is sometimes hard to gauge, so defer to the experts who know him, and you, best. You can ask his kindy/preschool teacher and speak with his new school, you'll probably find you're reassured and don't need to worry.