Sometimes we have to be hard to carry on. It changes us, but it can be a good thing as long as it doesn't harden our hearts.

I'm so sorry about the loss of your son and niece
I can't imagine the pain you went through when you lost your son...

I wish you could talk to your grandson about the loss of your son. It would benefit you both. But he grieves in his own way, and has chosen silence as his protective mechanism. His father's loss has hardened him, too.
I'm glad that you are able to have a relationship with your him even if that mean obeying his rules, and knowing that he won't accept the help he needs. It must be painful.