I started trumpet at 8 and took to it like a duck to water. And I wasn't a big kid for my age by any means. One thing I do remember from my final year at school is watching this little 11 year-old first-year climb onto the stage at a school concert lugging a trombone that was nearly the same length he was. 20 or so bars into Rimsky-Korsakov's Trombone Concerto, we knew we were listening to someone who was truly destined for stardom. His name was Ian Bousfield. Read the link. It gives a good insight into the learning curve and timeline common in the heartland of the British Brass Band movement. Their rule is "start 'em young". If your son is enthusiastic about learning trumpet, then my view is that you should grab that opportunity with both hands before interest wanes and he becomes distracted by other things.