B.W.
This is one of the toughest things for blended/step families. No matter how much love and support your son gets from his step-dad, he will always idolize his "real" dad -- even if the real dad were a schmuck. It's because he WANTS his real dad to be perfect. that's just the way it is. (And, yes, I am in a blended family, too)
There are a number of things going on here. For one, you and your son were, "me and you against the world" before you remarried. That got severed and he got displaced by a new spouse.
On top of that, the new spouse came with a new set of rules, and probably some unreal expectations of how children should behave. That's normal for someone who jumps into parenting mid-stream.
Add that to the normal reaction of idolizing the bio-parent and not wanting to like the step-parent, and it's kind of hard even in the best of times.
But what's most important is that the relationship that guides the family be in tact. You can't do anything to make your husband and son get along -- that's between the two of them, and most likely they are both mentally tearing their hair out -- one in frustration because the little guy is constantly challenging him, and the other because he just doesn't want that big guy in his life. And you get caught in the middle -- something your son is unconsciously working on. (it's called triangulation, and if he splits the two of you up, he regains a position of power in the family system -- it's routine with all children, even in families where both bio-parents are present, so don't blame your son for that. It's the old, Mom can I? no. go to dad, and say, "Dad can I?" thing)
So, for starters, work on the relationship with your spouse. The team at the top needs to be secure before anyone in the family has any security at all within the home. Plan some time away from all the kids, go to a park or something and walk around together, enjoy the scenery, and open your mouth and tell him that you are feeling pulled apart by what is going on at home. Don't BLAME him, he's probably doing his best, and he's frustrated with the situation. Be sure he knows you are frustrated, too, and you want your family unit to succeed. You want to stay together. (men are kind of fragile at this point, and if you sound at all critical, he'll withdraw or get angry to protect his emotions.)
See if together you can list the behaviors (not attitudes, not emotions, but BEHAVIORS) of your oldest that are positive and what is negative. Come up with a method of dealing with the negative behaviors that is fair and is something both of you agree on. Be a team. Agree on the course you set, and stick to it, evaluate it again in a month to see if it's helping or not.
And about emotions? They come and go. The times when your son returns from his dad's home are going to be the hardest, every single time -- his emotions and his sense of loss are going to be the most poignant when he's in transition. He is feeling torn apart, both when he leaves home to go to Dad's, and when he returns to not be with Dad. Be sure he is aware that you understand that, and give him extra emotional support and space during those interfaces. I used to HATE the first week after my kids returned from visiting any of my Ex-family. It was difficult at best, until everyone got back into a routine.
Make time to let him know that you care about his emotions, and his sense of loss (in 6 yr old logic, not adult words), AND that his behavior still needs to meet certain standards when he is at your house. You aren't disciplining him to be "mean", you love him, and you want him to behave in appropriate ways. That said, offer him something else to do -- perhaps build with legos in his own room, if he needs emotional space -- join him, without the step-bros and withoud the step-dad and build something together, because he'll be able to talk more when his hands are busy, than he would otherwise. If you try something like this, ask questions about things he did while he was at Dad's. No value judgements, just listen, or just build things without talking --even if you build separate items. Try to make his transition easier for him to deal with, and I think he'll be able to deal with the relationships in "your" home better than otherwise.
And allow him the freedom to idolize his dad. that's okay. Even if he's a schmuck. My oldest thinks the world of her dad even though he patently ignored her all through high school and most of college. His wife didn't like her. At all. The next one down really loves her step-dad and she just couldn't shovel the wrongs her bio-dad did under the carpet when he changed his tune and began to want to be a parent to the girls who were then 22 and 24. (kind of too late to my mind) But you know what ? He has cancer now, and while he's in chemo, I assume he is probably dying . . . and that puts all the ugly stuff in the realm of water over the dam -- it's gone, you can't change those years, you can't undo the wrongs . . but in the end, when someone is dying, they don't matter in the same way -- time changes lots of things, and at my ripe old age, looking backward, it's easy to say, hang in there . . .
What's hard is to work through the issues this situation brings between you and your husband. Try to keep those issues outside that circle -- find ways to enjoy each other, to live and love, and to struggle together, rather than let the circumstances in your lives divide you.
If necessary, build the plan together, but YOU take over disciplining your son. That way the discipline doesn't cause a divide between you and your husband, and it doesn't cause him to hate his step-dad -- his anger will be directed at you, but his relationship is stronger with you. Allow him and step-dad to build more positive ways of relating, so they respect each other prior to having step-dad disciplining him. (even if it's been happening for 6 years now) Remove the stumbling blocks, encourage them to do positive things together, and help them to respect each other -- then the discipline will become a more natural part of the relationship and the jealousy between the biological kids and the step-kid will begin to ease as well.
I wish you the best. It is a hard thing to deal with, but if you can create a team at the top, you will survive emotionally, and so wille everyone else. You will also avoid the heartache of having two such situations, because there will be 2 divorces. And that's no fun at all. This is a way big challenge, but it's surmountable. And when you've climbed the mountain and see the scenery from up there, it'll be WAY worth it. :-)