I have two boys, and since we read a lot the first son did this on his own on the potty chair. Daddy read in the bathroom, so he wanted to be big like daddy, he said.
It made sense to him, and I was pleased with his results.
I bought a roll of tiny stickers (flies, of all things) and gave him one each time he "went like a big boy". Ten years later, we still have some on the toilet brush holder.
Later, he closed the lid to the potty with a shield, and used it as a step stool to get up onto the big potty. We found seats that sat inside the big ones, but the shield was hard to move around for him. I decided to try the "dinky down" routine, which they managed well after some practice. (Kids are a bit chubby, so holding down the belly right over the dinky kept his hands out of the stream of pee.)
Another trick that might work for you, since potty happens in the tub, is warm water. Keep a cup just for potty time, and put warm water in it. Pour it over his dinky, viola, pee time.
Last thing we did, when the second son wanted to stand to go like daddy, I had to figure out something to minimize bad aim! (The first son was a natural aim, so this wasn't a problem for him.) He figured out that standing on his step stool and leaning forward (holding the back of the potty or opened seat lid) put things in proper position to not miss! (Mommy was thrilled!) Kids are inventive, so maybe my kids' ideas will work for you.
As for going poop, that was more cumbersome. When the first son was in training, I stopped changing him on his back and began having him stand. He wiggled, so we moved him to the vainity in the bathroom - right in front of the mirror. He had to stand with his hands on my shoulders, and I'd say, "Ooo, look what you put on yourself! Why'd you do that like a little baby boy? You're a big boy and I bet you'll be glad when you don't poop like that." The vainty didn't leave him much wiggle room, or an escape. Messes were easily be washed off with a washrag and kid soa. I intentionally used cool water (not cold). This worked really well for me because he had a little brother and didn't want to be a baby. I admit it took months, but I kept my mind on the final results.
(He reverted to bottle and pacy stealing, but that didn't last long either.)
The second son went easier since he had a big brother to mimic.
By the way, before complaining at any time that moms shouldn't have to help kids do these things, remember you have the opportunity to raise a young man who lifts the seat.