First things first, even if you are called as a witness at the tribunal, even if you are personally named on the claim, it is the company being sued not you personally, and even if you were found to be in some way at fault, the company should be liable for your actions, so please don't worry too much about any personal issues or needing to protect yourself, there shouldn't be a need to call ACAS or take personal legal advice (do so if it would reassure you of course).
Your company should hire a solicitor and/or barrister to manage the whole tribunal process and they should advise you and support you through the whole thing, what to expect etc. They may also be able to negotiate an out of court settlement (particularly if they think the company is likely to lose!) which would avoid the need for a hearing at all, so don't panic yet. I do think as line manager and first stage investigation manager you are fairly likely to be called as a witness if it does get as far as a court hearing though, so to let you know what to expect:
-I'm afraid it will be months before it gets to a hearing at all, and usually the first hearing is a case management/instructions hearing rather than the final/resolution hearing. It will probably be 6 months- a year before it is listed for final hearing due to court backlogs- on the plus side a lot of claimants drop out in the process particularly if they don't have a strong case so you may find it goes away by itself. As a witness you won't have to attend a case management hearing, your solicitor/HR will deal with that. I know it's easier said than done but try not to let it worry you in the meantime.
-Hearings are held in a court room, but as pp says it is more like a slightly large/fancy office or meeting room than a TV style/criminal court room with docks and audience galleries etc! It is however an adversarial process with barristers in wigs etc. Personally I have been a witness in tribunals many times and actually find it a bit less nerve wracking than an internal disciplinary meeting, because it is a very formal and controlled environment and you feel more distant from the actual person, but appreciate that is a personal thing...
-If you are called as a witness, your solicitor will work with you to draw up a witness statement which will form the main basis of your evidence. You will probably be asked to read this out in court and can be questioned on it by the judge and the opposing side's barrister - this is the scariest part but your solicitor/barrister should prepare you for the questions they are likely to ask and usually the process is very courteous - certainly the judge or your barrister will intervene to stop any aggressive or irrelevant questioning or rudeness etc. The statement and questioning will be limited to your role in the process so you shouldn't be asked why he was dismissed etc.
Hope this helps, do try to take things one step at a time, of course it is not good that he is sueing the company but if you have confidence you did everything by the book there's nothing for you to worry about and there's every chance it may not end up in court at all...