Our piano teacher will not even start any children until they understand left to right, top to bottom, these are lines, these are spaces, these dots get one beat, oh, you don't do math yet so you don't understand what a beat is....
Kids really can't start a formal piano lesson until they have basic understanding of counting and how to read and Spatial concepts. They just can't comprehend what is going on in the music.
Letting a child play on the piano isn't a bad thing of course but they are not cognitively able to understand music at all at this point.
I would call around and find out from professional teachers of the piano if there is any damage that this can do. Such as developing bad habits that will take a life time of lessons to break.
I know when I was an adult and took my first piano lesson I could not do several things correctly because when I would play a piece of music I made up my own fingering style and the piano teachers I had as an adult tried to break me of those habits but they were too deeply ingrained. I can't play the piano any better than before I started because I could not learn proper finger placement.
So that is why I caution you from sitting him down to learn piano at this time. Doing something playful but correct would be the right thing. SO I would only trust a professional piano teacher that usually teaches this age group to teach this child.
You don't want to enroll them in piano at 7 years of age and find out they are beyond being taught due to bad finger habits or they can't read music the right way because they taught themselves what the notes were but missed out how to learn chords or sight reading the notes going up and down the page.
