Yep, both the KTM and KX 65's are great choices. Just look it over like you would any bike. And make sure you have a KTM dealer fairly close (or mail order) if you're one of those dads who likes to keep everything in tip top shape.
But as far as the first questions, for being 9, and having mastered the 50, I think he will do well on a 65 - just takes a while to get used to the clutch and shifting, but he should get the hang of it. One of our good friend's daughter's is 8, and she just went from a CRF 50 to a KX 65 and is doing great with it.