K.H.
I know a few piano teachers and they have rules about starting piano lessons. I asked about it because i want to start my daughter early with lessons too... but the child needs to be able to read, and do basic math, and have a good attention span which is usually by first grade or 6 years old.
So, if he is reading, or close to it, start working on math and counting. Listening to music and playing musical games at home is also another thing they recommend to get them ready.