Very sorry to hear about your loss, no matter how much time has passed.
Like tamborine, I was about to say 'have you asked?'.
Maybe they haven't interred or scattered your son's ashes yet, but still have them at home.
As for cutting your grandchildren from your will - that has to be your decision, although they may not be aware of the heartache this has caused you. If it were me, I would write to ask where your son's ashes are, if you haven't already done this.
Hope you will soon find peace of mind....