sometimes it happenes to little boys when they need to pee. mine both have it first thing of a morning till they do there "morning wees"
also some boys go threw a stage were they "play" with it, just as its a new thing, but if you disscourage it, they grow out of it quick.

so dont worry its normal, nothing you have done!!
...so that's where MIL went wrong with DH. She didn't discourage him!