My son has been refusing milk for the past 3 days, he only drank a few ounces each day & prefers plain water. He is only 20 months!

Good... introduce him to different foods. It's a myth that milk is a complete meal. Earlier when there was not so much pollutants and other harmful factors, milk quality was good enough to be a complete meal for babies. Now everything that the animal eats, the environment that its kept in and all this is responsible for poor quality of milk. U will find cows and buffaloes feeding in trash areas and that's the filth that comes out in the milk. Try other foods....