Our DS was diagnosed with Selective Mutism a few years ago. Through intensive behavioral therapy he has progressed extremely well. He's involved in karate and baseball and interacts well with children and adults provided that the spotlight is not on him. If he feels he's getting too much attention he shut downs (only happens a few times a week now, compared to numerous times a day when first diagnosed). He is doing so well that his diagnosis was changed from selective mutism to social anxiety as he no longer met the criteria for sm.
As far as WDW goes we have gone twice a year for the past 3 or 4 years now. We started going when he was two or three. I suggest taking a stroller and getting the GAC, we couldn't have made it through those first couple of trips without it. The first trip he sat in the stroller at all times with the exception of riding rides. He did not talk to anyone and avoided all eye contact with cms. It made him feel safe and gave him his own personal space. The stamps we had on the GAC were the stroller as a wheelchair and alternative waiting area ones as he would have had a panic attack and subsequant meltdown if he had to be surrounded by all the other people waiting in line. For him he would rather sit in the stroller on the sidelines than be in the middle of that many people.
When we did (and still do) character meals he always sat in the inner most seat, furthest away from the aisle. Most of characters got it and left him alone, some did not. With those we gave a quick "no" shake of the head and they understood and would joke with him from the aisle. He still didn't respond, but they we're not in his space so he didn't completely shutdown. We also did a lot of prep work before the trip, watching the planning dvd, shows on the travel channel, etc so he knew what he could expect. We also watched movies with the characters so he knew who they were and that they'd be there. Basically we expected some road blocks during the first couple of trips. He did extremely well while we were there and for a weeks following our return home.
Hope that helps. If you have any other questions feel free to pm me.