SPD seating helps -

 - I could see making something like this with a small inflatable ball that is not fully inflated.

 - I couldn't link to the instructions as it sends me to all sorts of wacky ads. (SAD FACE).



OR
 - This idea with a tennis ball for the wood bench and swap it out with a softball for more difficulty - standing on the floor.



  
wobble board with rubber pad to keep it from slipping (Thanks Diane!)- RESULT _ he loved it, the wood bench took a beating.

Use a large Exercise Ball

T- Stool