I don’t think that normally the courts will allow a child to testify in court. If the case is for custody, the court may talk to the child but it would not necessarily be in open court.
If your son is so determined to do this, ask your attorney and allow that to be your guideline. Normally, siblings are not separated in custody cases, so you chances there are better than average. Maybe the attorney could talk to your son and see if it’s necessary to have his testimony.
It also sounds, to me at least, as though your son may need to say it just to get it out. He needs to tell it so maybe it would be a good idea for him to write out what he wants to say. It may make him feel better just getting it out and then his desire to go to court would be sated.
I don’t think that children understand enough everything that divorce and custody affect. Even though he wants to go and wants to talk to the court on your behalf, I don’t think it would be a good idea. I think it would be more stress on him than he realizes. My stepsons also wanted to go to court during the custody. They were upset that they couldn’t go. The problem comes in that a child wants to stand up for their parent. If you had no one else to testify to your ex’s behavior then your child may have been the only way to know. I think you have ample evidence of his behavior and plenty of character witnesses.
If it’s like my husband’s custody, the preliminary hearing told the attorney’s pretty much all they needed to know and caused his ex to settle without going back to court. Our months of planning and gathering information for one hour in court…the judge ruled to hear the case and left the custody the way it was in their agreement. Then when they settled, custody and property were pretty much left as the agreement had it also, except she got a little less…