Mark deserves better than this. I can only help to a point and then we have to get in "better eyes" than mine.
Until then,
Are you or Mark's parents in contact with the police? How often? Maybe someone here, a relative, can help Mark's parents by acting as a liason between them and the police if they need it. For some people talking to the police can set off a range of emotions that could stop you from asking questions or from hearing them completely.
There must have been newspaper articles. Do you have any that you can post?
I would gather every bit of information you have on Mark and his disappearance. Ask his parents to help and go back and try to remember from the time they heard he was missing till now (that is only if they are up to it), recall everything that's happened; telephone calls, meetings with the police, dates, times and places; anything and everything that belongs to this case. All relevant papers. Put it together so you have everything if you ever need to produce information. Post whatever you feel would be helpful. Everything plus whatever they need for the radio show. You'll never have to do this again! It's a one time thing! I'm sure it's the same information you need for the radio show. I just can't wait now that you've told me!!
I think I'm going to start reading about the Gay Community Activists in Portland, Oregon. Mark was one of them and was heading home from a bar that was known to cater to the Gay Community. I would think that they would want to be helpful. I can't contact anyone. If I find someone I think may be important or helpful to the case, I'll let you know. Talk to you soon. Good Luck.