My 3 yr. old is doing the same thing, only he took the dog's bowl, sat behind the desk,w/his pants on, and peed in it. I too am at my wits end. I have a 5 yr.old and never experienced anything like this. We tried using sticker charts and everything you hear of. We put him in diapers and really babied him,holding his feet up to wipe and clean him even after pee, really exaggerated. That lasted 2 days, he hated it. I can't say that this works or not but he has been dry all day! Let's hope I didn't jinx myself.
Re: the onset of this, consider if anything has changed in his environment that may have him a bit unhinged. There may not be anything of an acute nature, but it's worth considering.
On the behavior management front, set clear limits about use of the toilet. If he behaves otherwise, have him assist in the clean-up, and have him spend five minutes (quietly) in a chair as a time out. My guess is that this behavior will rapidly cease. And remember to talk with him about it a little, but only a little. You want to be sure to check in with him about what he thinks is going on. But don't dwell on it.