Hi K.,
Just a few sources of information on cow's milk I thought you might find helpful.
http://www.drlam.com/A3R_brief_in_doc_format/print/2003-N...
http://milk.elehost.com/html/why_does_calcuim_leave_the_b...
http://home.bluegrass.net/~jclark/calcium_foods.htm
http://www.safeminds.org/mercury/
I would personally recommend providing a rounded alternative, meaning, switching between soy, rice, almond, or other kinds of milks and fruit juices(though nut milks may be risky to give to a young child). Dairy (among other other animal products) is known to harbor hormones and chemicals given to the cow (animal) it came from... and with especially with dairy this also means concentrated amounts. Consuming dairy means that we are consuming growth hormones and all these other chemicals that are dosed for 1000+/- pound cattle. This can cause all kinds of problems for us because they interact and interfere with our own hormone production. (A good example is women who typically suffer extreme menstrual cramps and complications, who go on a primarily vegan diet -- excluding specifically cow, pork, dairy, chicken, and other dairy products -- end up having significantly less pain/discomfort, or no pain at all during their periods. Hormones also impact breast tenderness and swelling during the monthly cycle and may contribute to fibrocystic breast disease. Male hormones are no less susceptible to the hormone interference.)
Also, make sure he's getting enough water in his diet (plain water), and 100% fruits and vegetables (meaning no additives like sugars/corn syrups).
Best wishes,
H.