On a general note, encourage him to do whatever he'd like to do. You wouldn't stop your little girl from playing football, right?
On a specific ballet note - no way in hell will any of my (future) children be doing ballet. I did ballet from a very young age - I did well at it, saw many tutors and teachers and professionals. Who all said I shouldn't bother carrying on past the age of eight, as they could all tell I wouldn't grow into the "correct" shape. I was watching Dancing on Ice the other night, and the Hollyoaks girl said she had pretty much the same experience - only they said she was overweight as a child and shouldn't bother carrying on.
Even though I was just doing it for fun and had no thoughts about being a professional, this absolutely devastated me.
They put children in pointe way, way, way too early and regardless of what kind of parent you have (in other words, not the pushy stage parents), the teachers generally tend to be adhering to that very specialised body-fascist ballet type.
An ex, whose mother was an ex professional ballerina, had similar stories - of children being x-rayed at five years old and turned away because they wouldn't grow into the right kind of body shape.
There's no way I'd want to expose my children to that kind of absurd pressure and criticism. I'm sure all your children have lovely teachers, I couldn't possibly say otherwise...
But I had several teachers, in different cities, and they were all the same: demanding, critical, bullying and putting ideas into my young head about body shapes that I should not have been exposed to. My parents had absolutely no idea until the "professionals" rejected me for my wrong body type. My rib cage would grow too wide, apparently 