If you think he is ready then I would try. I had someone telling me the other day their kid was driving them crazy with the "I'm bored" but he was a bit too young to start school at the beginning of the year, now that it's halfway through the year she can tell he is definitely ready to go!

ETA: I started school when I was 4 1/2 (so a year younger than everyone else) and (not to sound up myself or anything!) did really well at school and ended up with a great UAI and HSC results. I'm definitely not a leader, but obviously starting a bit early did me no harm.