What kind of Halloween Decor in particular scares him? I know you mentioned skeletons and howling...but scary/gorey/realistic skeletons or "funny" happy/friendly skeletons or any skeletons at all? I don't really remeber any skeletons (at least not any particularly gruesome ones), but their is definitly howling at the Haunted Mansion (and it can be heard through out most of Liberty Square).
I'd definitly try and avoid the Haunted Mansion area (you'd have to walk a little more, but you could completely avoid the area, that night) and maybe skip the parade (there can be some scary things such as the haunted mansion grave diggers with their very loud shovels.). (If you end up going through the Liberty Sqaure/Haunted Mansion area...maybe have your son use ear plugs?)
Mickey's Not So Scary Halloween party,
is not intended to be scary to children .... but it still terrifies some little ones. Just depends on the kid...
Does your son usually go trick or treating? Does he get scared? If he does, how is that situation handled? If he doesn't get scared, why?
Unfortunatly, there is a good chance that he will get scared at least once that night

. So definitly be prepared for that situation. (maybe look at a map ahead of time to see which counter service restraunts will be open in which areas of the park, it can be a good idea to quickly duck into Pecos Bill or Cosmic Rays for a well-lit, undecorated, calm place to relax for a couple of minutes). (or if he has some small *safety* item, maybe a small blankie or stuffed animal or something, carry that with you and pull it out if the need arises)
Try to get to the party early (I'm not sure what time the sun will set), so your son can see the decorations in day light...everything's always scarier in the dark.
Also, as a cast member I got to work several
MNSSHP's, and saw a couple of smart things that parents did for their kids.
One family I talked to dressed their son up as a "brave knight" with plastic armor. (they told him that the armor would protect him against any "bad" halloween ghoulies). The child (about 5, I belive) seemed very happy, and not at all scared. (Of course I think the big bag of candy he had collected might have had something to do with that also.

)
One night, working at a masking decorating location in Adventureland, a scared little girl was convinced, by my fellow cast member, that she could decorate a "special" "protective" mask. Then the rest of the night, whenever she got scared she could just raise the mask to her face and it would protect her from whatever scared her. I'm not sure that I really explained that well...but hopefully my point came across. Apparently it worked, because toward the end of the night her parents came over and thanked the cast member for the great idea.
Also, I once talked to a family waiting for the jungle cruise with two young children (both under 7) who apparently were scared of most halloween/scary things. The family (there were 4 of them) were all dressed as dwarfs (Sleepy, Dopey, Bashful and Grumpy) - but the awsome thing was that the parents had invited a couple they were friends with, who came dressed as Mr. and Mrs Incredible (from the movie). (Apparently the parents had mentioned their fear that the kids would be scared to the couple, before the event) As soon as the two groups met that night "Mr and Mrs Incredible" apparently sat down with the kids and told them they were there to protect them that night from any Halloween "badies". While talking to them, the kids were very proud to introduce me to their personal "body gaurds" for the night.

(this would definitly take some pre-planning but could be adapted...to most groups... maybe just have mom or dad go as a super hero?)
Above all, (I'm sure you already do this, but I think it bears repeating, especially if other people read this thread looking for advice), before going to the party try to remind your DS that it's all pretend, and that nothing there can hurt him.