Definitely think about getting him a fitty! My girls all started w/50's... now outgrown them a little. Got one on a 70, another on a KLX110L <---Very cool bike btw!
We got the whole family dirt bikes last year, including the wife. Great family fun! And the dirt biking will help your street/track riding immensely!
And if you pick up a used 50 and eventually a 70, you really won't loose too much money on them... they kinda quit depreciating by much - too much demand for them. Which btw, means you can throw NADA right out the window when looking for one.
BTW, forget the 50 for you... go w/the KLX110L (thank me later)