My first husband hit the road when my daughter was an infant. She expressed an interest in knowing him when she was 10. I found him for her (he was in prison - for child molestation of all things- you can't make this stuff up). they wrote to each other for a year. After than time, she said, "You know mom, he's kind of a jerk. I don't want to write to him any more. Charlie is my father now." charlie is my 2nd husband. I had not polluted her mind with any negative stuff about her real dad up to that point. she figured it out.
But if your grandson has a need to make his father into a hero, nothing you do will change that. Let it be.