We have tried all kinds of 'potty seats'--the all in one type my son wouldn't sit on whatsoever! We had to go with the seats that just go on top of the adult toilet. In reality, it is so much easier that way.
I did have to buy two the 1st one I got at Walmart for like 10 bucks. It worked fine and the kid loved it. But oh my, the urine smell was ridiculous and no matter how I had him 'point' it the urine got trapped in between the two seat parts, so I was cleaning just as if it was an all in one toilet.
So I bit the bullet and paid the enormous price for the Bjorn toilet trainer. In my opinion, it was sooo worth it. Its adjustable so it fits our oblong toilet seats in the other bathrooms. He can remove it and put back on his own without feeling like its moving! Wonderful, and its all one piece so there is no way for the urine to collect= no nasty urine smell! Can't say enough good things about it. Should have bought it the first time around could have saved myself some serious cash!
GL and I hope you find something that works for your lil guy!