You raised him like your own son since he was 4. And he seems to treat you as his mother even though you are no longer with his dad, and wants you (or needs you) there on the special day. I have no question in my mind that irrespective of your equation with his dad, you should go

That would be the right thing to do. You broke up with his dad, not with him. He can still be your son.
If you run into his dad just say hello and be civilized. I am sure you wouldn't feel good about it if you choose not to go. Go there, support the son, and be cool about the whole thing. And congratulations to you on the good news!