I don't think any human being needs milk. Clearly, our society survived for milennia before figuring out how to milk other species. Much of the world thrives with little to no dairy in their diets.
That doesn't mean that I think all humans should, therefore, abstain from drinking it. It's a culturally important food product, and has incredible emotional and social connections that are hard to deny. Further, while it's certainly not the "perfect" food that the dairy industry would have us believe (especially in the horribly damaged form they sell to us, pasteurized and homogenized), it is a nutritionally dense food. In our current environment of processed, severely nutritionally diminished foods (i.e. juice and crackers), I don't think that it's wise to disparage one of the few nutritionally dense foods that remains readily and inexpensively available to us just because it's imperfect.
My opinion is, while cow's/goat's milk is not *neccisary* for anyone, the one group that most benefits from it is toddlers who are not still nursing. I believe that toddlers need a lot of cholesterol, efa's and minerals. If you're still nursing your son, I don't think he needs supplemental milk at all. He gets plenty of good stuff from your breastmilk. I don't feel it would hurt to give him some, unless he turns out to be sensitive to it; but I don't think he needs it. For a toddler who is weaned, though, milk is a very effective way to deliver those factors, especially within the constraints of a vegetarian diet.