I would suggest doing a summer or semester of piano lessons with the piano teacher to see if piano clicks for your son. People take to one instrument better than another. Piano is one of the most difficult instruments to be emotionally expressive with, so some people don't care for it. However, it's relatively easy to learn at a beginner level.
Violin (or fiddle) are the same instrument, with variations of techniques and other nuances. At the beginning level the instruction would be the same. Violin requires more hands-on teaching at the beginning. It's not a do-it-yourself instrument. This is a situation where you could find a local teacher for violin for a couple years before going to a teacher for the fiddle technique.
I'm from a musical family - my parents were both piano teachers, my dad is a piano technician and they have a piano store too! I started learning violin about 2 years old but piano is my instrument. My parents did not want me to become a musician (for financial fears) so I had limited lessons. I played without instruction. I have repetitive stress injuries to my hands, wrists, thumb, shoulders, and jaw from self-teaching bad physical habits while playing musical instruments.
For this reason, I very strongly caution against staying with a lesser-trained music teacher for more than a semester or year. I think assessing the financial risk is wise before committing to spending thousands of dollars on musical instruments and lessons, but whenever possible, get the most highly trained teacher possible.
If your son plays past the first book, he will NEED a real piano to practice on. Not just any piano. Don't buy a used piano from an individual - you'll get ripped off! Used pianos lose their value faster than used cars! Don't forget they need tuning once or twice a year (depending on climate)! They're expensive (but a cheap one is not wise!). I'd suggest trying to find a piano for him to practice on that you don't have to buy. Universities have practice rooms with pianos. Churches often have pianos. His school might have a practice room with a piano. Community Center? Nursing Home?
