We regularly play the piano here - in fact, I struggle to get my own practice in because when I want to play is exactly the moment when he wants to play! But I don't mind too much; I'm glad that he likes it so much.
The hope is that he'll keep on enjoying it and eventually be quite good at it. So far, "practicing" isn't a problem.
And someday when he asks when he started to play the piano we'll just tell him that he's pretty much always done it. At least, I hope so. The piano teacher in me can't help but hope that he loves it as much as I do. Looks like we're off to a good start!