It sounds like your son feels that everything is pre-destined… that God has decided exactly how everything is going to work and that is the way it is. The problem with this thought is it can make a person feel that they have no control, that it doesn’t matter what they do (that was what God planned for them to do) and that provides them an excuse for everything. It can make people very bitter, or to make poor choices as they feel that it doesn’t matter. I’m not explaining it well.
From what you said, I don’t think that your son doesn’t care about people or doesn’t have compassion – he just doesn’t see how his prayer would change anything because he has in his mind that God has already decided.
I’m not sure how to explain how I think it really works… I feel that we have “agency” or the ability to choose, and that our choices matter. That said, I think that God does know ahead of time how we will choose – but that doesn’t take away our ability to choose. I feel that God either knows us all so well that he knows how everything will turn out, or that time is “different” for God. In this viewpoint, our choices DO matter. I kind of see it more like a knowledgeable parent. Because I know my children, I know what their choices would be in some situations (I don’t know them as well as God does…) – but that doesn’t take away the validity of their choice, or the consequences that may or may not follow.
So now, how does it work with asking for help in prayers? There are many places in the scriptures (I’m not going to look up references) where we are told that we must ask. “Ask and ye shall receive.”. I see it like a loving parent that sees us working hard at a task, but for our own growth, waits until we ask before helping us.
Now, how to explain that all to a 7yo? Not sure… as I’m not sure I’ve even made it clear for you guys!