I really hope you rethink this. Information like this is too much for a small child. You need to be the grown up.

You can say things like ;"I wish I could see you everyday" "I have always wanted to be your dad and it's been hard to see you because we live so far away. Also sometimes people can't get along and that happened to your mom and I. I loved you and love you more then I can explain. I really want you to know that I will always be here for you and that I am proud to be your dad. Now take him outside and show him how grand the universe is and tell him that you are seeing the same moon, sun and stars as he is and that whenever you look up you think of him seeing the same things.

Pick a story book that speaks to always being forever together and read it to him everytime you see him. DO NOT SAY BAD THINGS ABOUT HIS MOM! You can hurt your son if you are too interested in truth and justice and not his wellbeing. He needs a loving papa.