ANDY'S ANSWER: Tell your grandson to walk or sit with a friend at all times, because bullies tend to target kids that are alone. Enlist the help of other children and ask them to all stay together and walk as a group when they are not in the presence of adults.
Also, you can quietly talk to the school bus driver and let them know that your grandson is scared and needs to sit close to the front, and will they please keep an eye on him.
You will also need to re-address the problem to the school. If you don't, they will assume that it has been resolved. If the bullying is not reported, then no one can help.