I would recommend thinking of cow's milk and formula as the same in terms of volume. So if he currently drinks 3 bottles of formula, then expect him to probably drink 3 bottles or cup fulls of milk/formula. We switched completely to cow's milk at 1, but if you want to continue to use formula while also adding in cow's milk, then the quantity would be the same if that makes sense. So if he currently has 3 bottles of formula, you might offer a cup/bottle of cow's milk in the morning and then 2 cups/bottles of formula in the afternoon and at bedtime - or whatever amount you usually do. The amounts will gradually decrease over time anyway and then you can make a decision if you switch completely to cow's milk or continue to use formula (you don't need to use formula, btw, and there's no real benefit to it if you'd rather use cow's milk). By 14 months, my daughter was only having a cup of cow's milk at bedtime, then the rest of her dairy she gets in porridge/cereal or yogurt during the day.