I think what needs to happen first is that you get support. No one person or one family could deal with all of this alone.
This is what I might think about doing, not necessarily in order:
I would try to find a counselor who can help you problem solve and to put into place a plan.
I don't know what your financial resources are but I would think about an attorney. It doesn't seem right that your husband have complete responsibility for a child who is not his. There is a way to renounce parental rights. This is often the past for a child whose issues and needs vastly exceed what a family can provide.
Get him evaluated at the Child Development Center of a regional Children's hospital. This will include a neuropsychological exam and also consults with a psychiatrist and social worker. If there is not a Children's Hospital near you, consider a University Medical School. What these evaluations will do is give you a workable diagnosis and recommendations so that you know what you are dealing with. You can also use these to obtain special education services, which can include non public school or even residential treatment center placement.
I would consider therapy. None of this is your fault. Scapegoating you may be appealing (and even self-blame) but this will only makes this worse. It's not your fault. Nor should anybody make this about your changing, doing different, more, etc. This is a smokescreen. Try not to take it on.
I would consider Al Anon. This group will help you deal with the family systems stuff, to make boundaries and to not take on what is not yours.
How much support are you getting from your husband? None of this is yours to take on. Is he clear on this? You can choose to take on some of this, but how is this really about you? Your own mental health, peace of mind, serenity and functioning are essential and important, too. Your responsibility, first, is to make sure that you are intact and your needs are met, and that any child you brought into this marriage, is psychologically and emotionally and physically safe too. If this environment is toxic to you and to your child, this has to be factor number one. Only if you and your child are safe, not under undue strain, not living in constant stress and conflict, can you begin to start thinking of anything more.
I know how scary and sad it is to think about needing to make adjustments to a living arrangement, or a relationship. But if there is not safety, and the sense of safety, what real option is there?