And piano sounds like a great idea!
It does sound like a great idea. Just keep in mind that if your DS does have dyslexia and/or dysgraphia it will NOT come easily. We pushed DD through two years of piano and it was a toss up as to who suffered more ;-) Dyslexia makes it very hard to read music and dysgraphia makes it hard to control your fingers. In theory, it sounds like a great exercise for all of those weaknesses. In reality, be prepared that it may be yet another battle ground.
(And I am soooooo pro-piano otherwise... I have my grade 9 RC, put myself through university by teaching and played piano/organ at church for 16 years. It broke my heart, but I had to let DD stop lessons)
