potty training is he ready?

my boy sits on the potty happily with nappy on..........you dont have to tell him if he sees it he sits on it...........(we keep it in the lounge) he also knows wat the toilet paper is used for and that we put it in the loo after...............but after the shower we put him on he sits there for a bit then stands up and pees on the ground..............at the moment im not moving fast enough to put it under him..............how do i get him to do something on there? short of my husband sitting on it to show him....which would be amazing but i think his butt may get stuck. :)