In terms of the 9 yr. old: If you think he is mature enough, it may do you well to explain some things to him, like why you think that the older ones act that way. It might be a good opportunity to show him how to be understanding and empathetic of others (a good life skill). It would open a conversation in which you explain that it's not his fault how he is being treated, and that sometimes people take anger about an issue out on other people. I don't know how much about their lives you would want to tell him, but even just explaining that they are going through some problems right now, and that even though they are not dealing with it constructively, that they are just acting out anger, it isn't his fault, and that even though they are not acting like it, they do love him. Sometimes just explaining to kids why things are going on is enough to help them get through something. I would emphasize, however, that this is not an appropriate way to deal with problems, and maybe even give him some examples of how to do it appropriately.

In terms of the fairness regarding discipline of the older two: It sounds like you and he have talked about this before. You said you have come a long way, so maybe some more talking is in order. The thing about people is that they don't always like to be told that they are part of a problem that is hurting their family. Some folks get defensive and refuse to be part of the solution.

Maybe you could focus on the improvements that have been made and what it has done for your family. Then, based on the positive effects, note the issues about the discipline. Just make sure that you focus on "you're a good father, but you could be an even better father" as opposed to "you are doing this wrong". You may consider the idea, as well, that maybe he feels like the older ones should be approached/treated/disciplined differently than the younger ones due to the age difference (which could be a valid point). If this is the case, you two need to talk about it. Make sure you understand where he is coming from, and then you could talk about more appropriate ways of dealing with them. Maybe he just needs a better strategy. This is something you should agree on.

Let your son stand up for himself to them. Not physically but with his mouth. He does not have to put up with it. And if your husband has a problem with it then maybe he should start doing his job. Irregardless those kids are all of yours and should be treated equally. At least with the respect on discipline. Also if your husband wants to be so lenient on his angles.. then maybe tell him that maybe you should both back off and just parent each others kids on your own. Like you discipline yours and he disciplines his..

Just a thought..
Happy

" make sure you tell the people you love most EVERYDAY.. Its important not only for them to know but for you to tell.. Life is to short to be miserable..