Yikes... I would think that he was doing it by accident, but the fact that he is peeing in a garbage can says otherwise. This seems to be a willfull act. I like the other suggestions; getting your son to use a different washroom, and supervising his activities (although that is kind of crappy for you, now you have to take the time to stand and watch).
If this continued, I would start implementing consequences. Try to get in there as soon as he is done and check. If he made a mess; do whatever you do for discipline (time out, loss of privileges, etc).
That is unacceptable to me. And I am raising boys, so I am used to dealing with a little bit of "pee spray". LoL