Judthepud, I agree with much of what has been suggested in most of the posts, although I don’t think I’d role play the possible scenario as it could be seen as coaching him in an undesirable way. I think you should support him as much as possible to find his father, don’t say anything negative about him, and find a neutral place to meet if you are successful in finding him. But do not leave him with his father, just stay in the background but don’t leave whichever building they are in.
One of my children has an adopted child, who is too young to ask too much at present, but we are all aware that the day may well come, and how we need to support our little one to find the natural family, and let him discover these people for themselves. Our opinions on the natural family don’t matter, although from what we know, it’s unlikely that any of us would meet in the normal way of events. I guess your DG’s father might well fall into that category. Remember he has had positive role models (male) in his life and he therefore knows how a reasonable and caring adult male should behave.