As you may imagine, I've been called to court numerous times. As a guess I'd say about 30 or 40 times? It may therefore surprise you to know that I've only actually had to give evidence about 4 or 5 times. Most of the time they plead not guilty, hoping to strike a deal on the day, or hoping witnesses don't turn up.
If you want a prediction for your job, I predict that he will plead guilty to a lesser charge on the day (do they have the public order act in Scotland?) and you won't even get called.
If you do get called, it's nothing to worry about - honestly! The lawyer on 'your side' (in this case I think the prosecution) will normally walk you through what they want you to say with mostly 'yes/no' answers - I. E. They say "is it true on 12th October 2014 you were in Pretty Park with your husband?" rather than "Where were you? Who were you with?". This is usually only not true when they want more detail and can't lead you - I. E. "and how did that make you feel?"
If you can answer a question with "yes" or "no" then do so.
When the defence ask their questions, they normally want one of two things - they want to discredit your evidence, or they want you to confirm something that assists their defence.
If they want you to assist their defence, it's easy - just be honest, and don't over think anything - listen carefully to the question and be truthful. Its for the court to decide on the whole picture later. "Is it true when you arrived that my client wasn't holding the knife?" "Yes."
If they want to undermine you, they won't be as nasty as the movies portray. They usually just ask the same question a few times to see if they can get any hesitation from you "let me thank you for being here - I'm sure it was a stressful situation am I right? You didn't seem sure when you said you saw him kick him three times - are you 100 percent convinced?" "are you sure it couldn't have been twice? Is there anything else you may have got wrong because of the stress you felt? " Just be a stuck record and don't let them sway you from what you know the truth is. When you've calmly answered the same question three times the judge /magistrate tends to speak up and ask them to move on.
Your Aspergers would qualify you in English courts for consideration of special measures, so it's worth contacting them to ask about that.
Good luck, let us know how it goes, and if no one on the day says it - thank you. Without people being willing to stand up for the rights of society, our society lets the guilty go free to do the same again - so genuinely, well done.