OP
Your story is also his story. You can see your own pain, powerlessness, loss, need to exist beyond the loss, frustration that somebody isn’t trying hard enough to untangle a huge historical web of collapsed parental bonds.
Shift the lens. Go through each stage and consider his perspective with the same sense of degree of powerlessness, loss and confusion you factor in for yourself.
DIL can’t fix this. She wouldn’t even know where to start. Don’t fix your sights on her as the one responsible to unpick this. If a spouse could or should do that, then why did your late husband not do it ? You will have good reason to understand why he could not be expected to do this. Allow those same reasons to help you understand why she cannot be the solution you expect her to be.
Your single greatest tool right now is the capacity to view the events of the past from perspectives other than your own. As hard as it can be to recognise that other people also have their limitations, inability to cope, mental & emotional struggles, and that they are more than “the bad one” in the equation, it’s still what you need to do.
Because if you come at your son with only your lens colouring your view of who did what wrong and who should be putting things right you are far more likely to cement his distance from you than make him feel you are safe enough to allow in even a little bit.
You have self compassion for how hard it was for you to fight for a relationship with your son over the years. Can you afford him the same compassion that his “able & ready now” state is not running on the same timeline as yours ?
I’d also caution against taking a very black and white view of his father’s choices. At the very least in any communications with your son. It is perfectly possible to think your parent made awful choices, become estranged from them as a result, and still deeply resent criticism of them by the parent who wasn’t there.
Nobody can tell you how this will end. Only you know how much time, energy and personal cost you are prepared to pay in the attempt to reconnect with your son. But you can establish a start point that relies exclusively on your willingness to try.
That start point is practicing going over the past events with the perspective of the other people, adults and child, who were involved. It’s not at all easy. But based on my own experience (albeit as the child not the parent), stepping into the “shades of grey” skin of ALL the others involved gives a much more nuanced picture of how things evolved into the final mess.
I ran out of time before I ran out of hope. But despite the lack of Disney happy ending, I finally found a place of peace. If nothing else it is an exercise that may help you process your pain at how the past panned out. So worth a try on that basis alone.