It sounds as though it’s a pretty open and shut case. There’s no question that your grandson is responsible for the death of another person. I think the PD is doing the best they can: plea bargain and hope for a lighter sentence.
A lot will depend on previous convictions, if any, arrests, etc.
I understand this is incredibly painful for you, but he’s a 23 yo man, not a young boy, and he’s responsible for the loss of a life. You can’t save him from the consequences of his actions. Let the PD handle it. Hiring someone isn’t going to solve this, and takes a massive gamble in going to trial. You could spend tens of thousands of dollars, only for your grandson to end up in a worse situation than a plea bargain could be, and you could end up bankrupt with the bills.