From a Pastor's perspective, when someone says they have decided, they have decided. I treat it as such. I don't try to change their minds.
Of course, no one has ever told me they were gonna murder someone or commit suicide so don't be a dork. ok?
To carry this further, if someone comes to me and says, "I have decided to divorce my wife, what do you think?" I tell them what I think doesn't really matter if they have already decided. Depending on their response, I will know how to proceed.
A normal response I have seen over many years is they will blink their eyes about 3 times, stare and not move. I slowly walk away at that point. About 1/3 will come back to discuss their issues and about 2/3 stick with their plan.
Now, if they come to me seeking advice then everything changes. We meet as couples, with my wife there as well, and have discussions. I generally take them to dinner.
Since he didn't ask you for advice, I wouldn't give any at this point in time. The only question I would ask him first is how are you gonna deal with your 6 year old?
He'll come to you for advice when he's ready. Since you haven't been married, it may be out of your wheel house to give much advice. I don't know anything about you so I am speaking in general terms. You may be a trained marriage counselor, I don't know.
Do not take sides. Don't let him talk bad about his child's mother. Especially in front of him/her.
Be careful of your body language when he is telling you his side. Don't be nodding or shaking your head in response to his remarks. Just be still and listen. Be stoic and as unemotional as you can.
I don't know your beliefs, but I would pray with him quite a bit. Pray for wisdom. There is only One who can and does change hearts.