S.L.
Above all, just have patience. Understanding the concept is one thing. But understanding the muscles that make things happen is another. When a child first sits on the toilet they actually try and hold it in. It feels different to just let go hanging over the toilet. They are so used to the diaper being there to catch it.
When I start with a child of my own I start with putting them on every 20 minutes until they have gone in the toilet at least 3-4 times. Then I increase to 30 minutes, 45 minutes, an hour etc. But at some point it has to be them wanting to go and not just you putting him on. So just don't expect a lot too fast. It will happen.
Suzi