my son only use the digital piano for very short time, if your son will learning the piano and your house have extra space for place the piano, I think you can think about to buy him a second hand piano.
Thank you very much for reply our question, you are very kindlly and helpfull.
Now My son's piano teacher is using the following books.
1. John W. Schaum
2. Sep by step Piano Course Book Four
3. John Thompson's modern course for the piano second grade book
4. Scales & Arpeggios Grade 2
5. Everybody book two
6. My first Theory book
His teacher help him to preparing Grade 2 exam next year.
do you thing those books can help him and anything else I need to considerate?