M.S.
Hi H.-
You mentioned that the 2 parents of this 8 yr old boy are not taking more responsibility for their own child. You also mention that he needs more from a caring person then he is getting from either parent. So with that said, by you, this is my response.......
Are you married to his father? Or are you just in a relationship with him? If you are just in a relationship with the father, then ask yourself why you want to continue to be involved with him, when you have just pointed out that he needs to be more responsible and caring with his own son.
If you are married you will now play a role in this little boy's life as a step-mom. I'm not sure why you don't know what to say to him, or why your concerned about being an awful step-parent. The best thing you can do for this child when he is with you is show him love and support. Greet him each day after school with a snack and a hug, ask him about his day, help him with homework. Cook dinner in the evenings (since you are not working and our home now) and make sure the entire family eats dinner together (if possible). After dinner ask him to help you clean up, enjoy some playtime after dinner, bath time and then off to bed by 8:30pm (obviously just my opinion).
Children despreately NEED love, stability, and consistency. The best thing you can do for him is provide consistency in his life. I have a 10 yr old step-son who lives with us full time and a 2 yr old son with my husband. I love both of my boys very much. Despite what kind of biological mom my step-son has, doesn't effect the relationship i have with him or the love i have for him. Just try and remember that it is not the child's fault what kind of irresponsible parents he has and it should never be taken out on him in any way. The worst thing anyone can do is talk badly about a child's parents to the child.
Pray, pray for this little boy and also (if your not married yet) ask God to show you which direction in life he wants you to take. Especially if you have always felt and maybe find yourself in situations with too much responsibility. God hears and answers the simpliest of prayers. With out God centered in your life, you will always find yourself in unbalanced situtations or relationships. Although this may be the pattern you seem to always fall into, it doesn't have to continue to be that way.
Take care-M.