Could you use permanent marker and draw a target on the bottom of the potty, maybe do 3 rings, a 1,2 & a 3... then on a piece of paper draw up a grid and every time he gets say the 3 (the middle one) he puts a sticker in the 3, once he has say 5 3's then he gets a prize?

The prize could be anything really, to play with a toy of his choosing for 30min, a biscuit, etc etc or get small tinkets from the cheap shop.. bubbles blah blah and he can choose 1!

No idea really... thought might be fun... lol and all kids love putting stickers on a grid!