Hi M.!
I'm a 34 yr. old mother of 2 girls, 11 and 9. I have the exact same problem with my 11 yr. old, which started at about the same age. I guess my take on it has always been that it is better to get some milk into them than not to have them drink any at all. I don't buy the prepared chocolate milk. Instead, I buy 2% white milk, then let my daughter use Ovaltine rather than Nestle Quik. It was a compromise, as chocolate milk has almost twice the calories, but at least Ovaltine has added vitamins and an extra 20% of the recommended daily amount of Iron. (My daughter also says it tastes the same as Quik) It is not ideal, but growing bones need calcium, and while a supplement is good, I still personally believe that it is better to get it from food. Also remember that there are other good sources of calcium, such as cheese, yogurt, and cottage cheese. If your son will eat these things, then the milk isn't quite as important. But I don't believe that it will hurt anything to let children have some chocolate milk each day. But be aware that this may spark your other children to want equal treatment, especially if they love chocolate milk too. Thankfully, my 9 year old LOVES white milk, so I don't have any problems. Hope this helps some.
S.