It's an excellent try, Jorge. In fact it's exactly right. 'Who is he disguised as?' is a perfectly correct and natural-sounding question. The only problem is with the word 'disguised'. In English, we only use the word 'disguise' is if your intention is to genuinely mislead someone into believing that you are not who you are. For example, a criminal might disguise himself to avoid arrest. If you are talking about Hallowe'en or party costumes, and you are wearing costumes just for fun, the expression is this: 'Who is he dressed up as?' 'Superman'. I hope that helps.

Perfect. This is THE natural way to say it. I'm completely confident in saying that. (I'm a native U.S. speaker but not a teacher). I will add two thoughts but don't trust me on these--see what the teachers say. 1) I believe that to be STRICTLY correct one would use "whom" rather than "who." 2) There is also a "grammar myth" that prohibits ending a sentence with a preposition (it's nonsense). Therefore, someone who was trying to use perfect grammar might write "As whom was he disguised?" To my ear, this sounds AWFUL--unnatural and affected.