S.H.
I was 5 when my dad died of leukemia. I think that the best thing you can do is try to provide as stable of an environment as possible. He's going to need some sort of normalcy and it's good that you have been babysitting him which will help with that. Some children at that age don't understand that a parent has died. He may ask for him and it's fine to talk to him about his father. If you are friends with his mom, be available for her to talk to. You can also ask her what you can help with pertaining to her son during this adjustment. Good luck and I hope this helps. My thoughts are with the little boy and his mom and they will get through this.