You didn't mention if he was upset about cleaning his room or if he did it while you were in the room...It's possible that he did it before you ever went into the room, right? So, I'll share this story with you:
I took my 10 year old clothes shopping. We were in the dressing room a while, but not "forever". When he finished in the dressing room, he came out with his sweatshirt wrapped around his waist. Eventually, I found out that he had wet his pants.
As you know, by the ages of our sons, they do NOT pee in their pants on purpose. He was extremely embarrassed. It was simply an accident - he had to pee, but wanted to finish the clothes thing, and then he had to go so BAD that it just came out.
So, my question to you is, could your son have had an incident like this where he was playing and then suddenly just had to go? Didn't want to wet on himself, but couldn't make it to the bathroom? Are you sure he was acting out?