Kat: He's the right age. We're waiting for our daughter to be ready. She was three on Valentine's Day and her mother is a fine fiddler.
I would worry more about the fiddle's ability to withstand the abuse more than anything else. Isn't 1/2 size a little big though? You may want to measure him for one. Also, most instruction schools will rent the fiddles out. Maybe save your money for the "Strad."