For background, I'm a 41 yr old SAHM with a step-son that's 8 and a son who's 2. It's taken a lot of support from my husband. I take my 8 yr old out on "dates" while my husband watches our other son. That way he feels special and not excluded. He has friends that play nicely with my 2 yr old. I make sure that he has a decent balance between playing with his brother and playing with only his friends. This doesn't mean that they don't have times when they don't get along, they seem to have less conflicts.
My 8 yr old also has "teenaged" attitude problems. We still do the groundings and take away TV/video game privileges. We also have him do time out or if it seems that doing something physical would help more, then I have him do push ups or jumping jacks. Lately, he's been showing his anger by tearing things up and making a mess in his room. My husband and I remind him that it's unacceptable behavior, let him calm down, have him clean up his room and have him work on my husband's punching bag. We let him know that it's okay to be angry or frustrated, but that his display of his emotion is the problem. Then, we offer an alternative or two that's acceptable. We let him pout for a bit, but we also let him know that he can't pout all day. Also, when he gives me the "that's not fair," I let him know that it's not about being fair. It's about consequences for not following the rules and being responsible and respectful. When he really throws a fit, then I let him know that I can treat him like his brother and have the constant attention, but that would mean diapers and sippy cups and all the other things that go along with being 2.
Overall, take time to talk to him and find out what's going on. Most of the time, I find that my 8 yr old doesn't feel like he's being heard. My 8yr old still throws attitude from time to time, but we at least have a plan of discipline that helps me keep my sanity.
I wish you luck and sanity.