I have tried these things 1. I got my son to sit on a yoga ball whilst listening.. it gave him some defeee of movement whilst sitting.
2. I alternated between sitting and standing. I put the laptop on a shelf that was at his height. Standing was better for him.
3. I used a sand timers beside the screen a 30min, 15min 10min 5min as I had these and started with the 30 mins saying he could turn them over one by one and when the last one had finished the lesson would be over.