You poor thing! I know the feeling of being helpless and the situation being out of control. He has got the better of you, and it's time to turn the situation around and get control. To start, I would tell him that he is not going to do that any more, and make him an offer he can't refuse (take some major privilege away - like not going swimming, or taking all his toys away...). It may take some time for you to convince him that you are serious, as he is testing you to see what will happen. As long as it is inconsistent, he will continue testing.
I would then be having him clean up the mess (as much as possible). Hold his hands with yours if you must and have him scrub. I would then clean him up with a cold bath. He needs to learn that his behavior is not going to be tolerated or rewarded (with loving praise, which is useless when it's time for correction), and I would be putting the baby to sleep somewhere else, so he doesn't have access.
Then, I would highly recommend reading John Rosemond. He is no nonsense and doesn't buy all the psycho babble of today. He has written a lot of books. For you, I would recommend Making the Terrible Twos Terrific (you're a bit past that, but it still applies). You can read some of his newspaper articles on www.rosemond.com, and get a feel for him. You can get his books there, or on amazon, or maybe try the library. He will teach you how to become a powerful, loving parent, who is in charge.
Once he is under control, it's time to potty train. The book Toilet Training in Less than a Day worked for us on our 27 month old, who has been in underwear for 5 months now. Good luck to you!