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?'
I hope that helps.