My grandson is 6 and in first grade. I opened a 529 plan for him in Oregon where we live on his first birthday.
I suggest you look into the 529 plan where you live. It is a good idea, as a grandparent, to get the 529 plan in your name with the grandson as the beneficiary because the money in the account will NOT be used to determine how much college aid he and his parents are eligible for. If the 529 is in the parents name then it is used when college aid is calculated. The 529 plan I'm in does not require a set amount every month. You can put in as much as you want whenever you want. And, the contributions are a tax credit on my state income tax bill.
Check into what the minimum contribution is for the 529. Whenever you or any of your grandson's family has a few bucks put in into the 529. That way you can take advantage of tax deferred compounding until he reaches college age. Whatever you can save for him will help. The worst thing to do is put this off until his teen years when the amount he needs will seem insurmountable.
It's great you are thinking about your grandson's education.