Sorry I can't help. I've actually never box trained a cat before. Or if I have it was so long ago I don't remember.
Cathy trained Paizly before I got her (Thank you Cathy!!), and Jax was trained when I got him too.
I know a friend had a kitten and they would just grab her and put her in the box every time she even looked like she was thinking about going potty. That's probably what I would do. And when you can't watch him keep him in the crate or bathroom with the box. But others have already said this.