Denial is unfortunately pretty common but most parents just need a bit of time and plenty of evidence before they come round to the idea that there's a problem.
Autism is a scary idea for many parents to wrap their brains around - it's a lifelong condition and yep.. it's scary for parents to allow the fact that their child will not be the child they were dreaming of but will be someone very different.. someone that they have no idea how to relate to yet.
Be gentle with them and if your grandson does have autism they will have to accept it in the end.. most likely they'll come round in the first year or so of school.
Curious.. what kind of intervention do you mean? Many children are diagnosed in their first few years of school and their futures can be really great as long as there's enough support available. I'm not sure i understand how an intervention now would change his future too much..