You should be introducing it to him now - the whole reason why we are supposed to put our children on whole cow's milk at 12 months is for brain development purposes, not just because someone arbitrarily made that rule. The fat in whole milk actually creates and insulates the myelin sheaths in the brain - in lemans terms, the fat content of the milk is what creates, connects, and holds together the rapidly growing and firing synapses that allow children to think and learn. There is not enough calcium or vitamins in your breastmilk to sustain his rapid growth: that doesn't mean you have to stop nursing him, but at this age, the nursing really provides very little nutrition and is more for comfort. A this age, the american pediatric association recommends a 12 month old child get "two cups" of milk a day which translates roughly to 12-16 oz a day max.