If you’re serious about this, I’d think about a few things first, think about your son and how he’ll feel, maybe speak to him privately BEFORE you change your will (assuming you already have one), is this just your money (I know it’s yours now, but did this come to you via a husband?), how would your husband have felt about this?
Finally, if you’re truly serious, and I think it’s an awful idea btw, consult a solicitor.
Please, please think very carefully about how this might affect the relationship between your grandson and his parents.
You do not mention where you live? If you’re in England you can literally leave your money to whomever you like, might not be quite so easy in other countries.
Have thought about leaving it split between your son and grandson?