He never liked milk much I just wonder if there is a health reason for this.
11 is about when boys often start growing to their adult size; often it's a bit later, but 11 is certainly within reasonable bounds for that.
Cravings for particular foods are often a sign that you need something in them. In the case of milk, you have a lot of things - some protein, some calcium, some vitamins, but mostly it's a rich, caloric drink. It's not necessarily better than a lot of foods, but it's a lot more nutritious than water! A lot of what is in milk is useful for bone and muscle development, both of which are going on at his age. (Whey protein, from milk, is a common drink additive for body builders, for example.)
Of course, if this is excessive or makes you uncomfortable, you should always speak with your son's doctor.