This is funny. I think there must be many reasons our little ones do this! My Maltese does the back leg kick after she poops, but only uses her back legs to kick, doesn't put her whole body into it like AprilLove does.
Road61, I think in time you will learn what your little guy is trying to say when he does it, if it isn't always after pottying.
PS. Not hijacking here, but "twank you Moosie" I wub you