He could carry a flip phone and keep an iPad, desktop, or laptop at home with which to keep connected. Unfortunately, some folks wouldn't keep in touch with others if they were living in the same house.
You're there for your kids; you can't force him into the fold, but you can learn from their behavior, and as the kids get older- well, the absence of grandpa makes for informative conversations on the importance of mental health.
It sounds to me like you're doing the best you can to make up for his absence; as father's in this situation it's all you can do. My kids are fortunate enough to have my wife's father as an elder male role-model, I hope your children are just as blessed.
Sorry if this took a bit of a turn for the personal, your post just hit me rather hard.