Welcome! I am sorry things are so rough right now. To be honest, the situation sounds like a powder keg waiting to go off.
It is NO ONE's fault. Physical violence simply CANNOT be tolerated, no matter the age of the violent person.
You have taken some very good steps, and it is wonderful that the boys have you in their lives. It is very clear in your post how much you love them.
I think that rather than wait for appointments, etc... I would take my violent child to a Children's Hospital for an inpatient evaluation. He sounds like he is way past the point of parenting changes fixing things.
It is hard to think about our little ones in a psychiatric hospital (psychiatric hospital). The kids seem to handle it very well. Some of htem are even comforted by the rigid schedules and rules. It is impossible to provide that level of care in the home.
It would be a good idea to think about this, make calls to your insurance company, etc... to see what is available, and think about it with husband. It may be what difficult child needs to get through his problems.
Gentle hugs for your mommy heart!