This might be common knowledge to everyone...but I "fixed" my problem.

When I rolled up first time, I was wearing the disguise and Halewyn opened the door. When I got to the "doing his "O Wind" dance" I wasn't. However, when I removed the cape, I was wearing the disguise. The sheep's skull was visible so I assumed I was in disguise.

When I reset and tried again from an earlier save, he wouldn't open the door...even though I was in disguise.

Except that I wasn't strictly in disguise. I had the skull on but wasn't in disguise. Only when I did the standard get into disguise from the down-arrow wheel, did it work.

So, not sure it'll help anyone else but even though you might be represented as wearing the skull, put the disguise on. It worked for me.

Just experiencing this bug.

First time attempt, wearing the mask he comes out, I light the mannequins. I view all clues but when I go back to him, there's no option to speak and I get the "get in disguise" message. I am in disguise.
Re-load an earlier save and now, he won't even asnwer the door regardless as to whether I am in disguise or not.

Yes - others are experiencing this. And bugs happen.

However, 4 or 5 months and you still haven't fixed it? Really?