Well, although this child does sound like he would test nearly anyone's patience I am struck by the fact that you are a self proclaimed yeller and find this to be an effective teaching method.  Do you do a task better, faster, with more clarity when someone yells at you?  Children require patience and a lot of it.  They are learning and it sounds like this child's wetting problem could be a learned behavior.  My suggestion would be to show him that there are both positive and negative consequences for his actions, but  do this with out losing your temper or yelling.  Speak to him.  Remember he is a person too, just because he isn't fully grown he still deserves respect.

I realize this can be easier said than done.  Of course I too have had students that I could have yelled at, but I think they were the ones that stretched me and helped me to grow both professionally and as a person.

Remember, maybe you're the only positive influence in his life.  You could be the one person that gives him any attention, any one on one adult conversation each day.  Don't miss the opportunity to use that for good.

I'm not trying to be rude, but come on....the kid wets his pants for crying out loud! Hey, don't feel bad- it's the end of the year and we are all on edge...I definitely would get tired of dealing with a child wetting his pants....