The other day when Ethan got out of the bathtub he was standing on the toilet before I put his diaper on him. He suddenly said "pee-pee" but it was too late. He showered the floor, but at least I knew he was becoming aware of when he has to go.
So yesterday I sat him on the toilet after he got out of the bath and he had a fun time singing about the potty for about 5 minutes before he went to the bathroom.
I don't know when you should start teaching boys to go standing up, but I think I'll stick with him sitting until I think he can aim. Knowing him it would become a big game and I would be cleaning every surface in the bathroom 5 times a day.