If you plan to continue breastfeeding, there is no need to start feeding your son cow's milk. My daughter's pediatrician asked at each visit starting at a year, if I had started my daughter on whole milk yet, and I said, "No, she is still breastfeeding." Her response was always, "Ok, great!"
Human milk is perfectly designed for baby humans! Cow's milk is perfectly designed for baby cows. There is nothing a child is missing out on by not drinking cow's milk, if they are breastfed.
Early introduction of cow's milk actually increases the chance of developing childhood diabetes (type-1). In Disease Proof Your Child, by Dr. Joel Fuhrman, he recommends that a child not be fed cow's milk, but that if used at all, not to start it before the child is 18 months old. He also says that after that, it is important not to use cow's milk exclusively, but to use an assortment of healthy drinks such as soy milk and nut milk.
Keep in mind that we are the only species that chooses to use another species' milk after weaned from our own kind.
My daughter is 33 months old, still nursing a couple times a day, and has never been fed cow's milk, or any other dairy product. She is growing quite nicely and is a very bright little girl with no allergies or health issues.
Good luck!