I don’t know what’s going on either but I do know that you are absolutely right to go for an evaluation. This is not normal sibling competition or boys-will-be-boys behavior. Your oldest is acting like a bully and it has to stop.
I’m wondering if you have ever sat down in a quiet moment and simply asked your oldest what’s up with him. Sometimes young kids surprise us by being quite candid. It’s also possible that he doesn’t know what comes over him to do these things either so don’t get angry if he can’t tell you. Just tell him you’re interested if he thinks of something and let it go.
Meanwhile, the most important thing you and the kids’ dad can do is be in complete agreement about consequences when the kids hurt each other. They are still at an age where parental approval generally means a lot. Sit them all down (yes, even the youngest) and calmly and certainly let them know that the rule of the house is that absolutely no one has the right to threaten or hurt anyone – ever. Be clear what the consequences will be. Messages delivered in a matter of fact, no nonsense tone are generally more impressive to kids than messages delivered in anger. Kids know when we mean it and they usually respond. You may even find that the oldest is relieved to have you assert some control over him. Hopefully, things will settle down at least until you get a full evaluation.
I wish you well.