Not to freak any of you out, but I was recently reading my local paper and saw an ad/request for participants in a new study from a drug company (can't remember which one right now though). Apparently there is a new "condition" or "syndrome" or whatever the heck they call them these days in which boys can begin puberty as early as age 3! I was stunned myself! I agree with the others who recommended taking your son to the doctor ASAP.
I too believe it is all the growth hormones in food these days. So an organic diet my be helpful, but I would definately seek medical advice immediately.