my son is almost 2 1/2 and we have the same problem. I found one seat that sits on top of the big toilet, but the guard that was up to prevent him from peeing straight out almost ripped his "goods" off trying to get up on the toilet (the guard was almost square and very tall). So, I bought a different seat that you put on the big toilet the the guard was a little shorter and round, but from the position he sits on the toilet the pee would have hit the guard and splashed right back at him. So, I think the solution is to sit him back on the big toilet and tell him to open his legs wide and that positions the "goods" at a perfecet aim. He will eventually hold himself up on his own. Another option we tried, and is working great, is the Peter Potty. It's a little urinal. At first I thught it was a big mistake. He thought it was a toy at first, then refused to use it, then all of a sudden he's pulling his pull-up down and going by himself. They have to stand REAL close to it, though. Oh, like a previous poster stated, sitting him backwards on the toilet works well too.
good luck