My husband and my son are both not milk drinkers!! My son is 6yo and drinks milk occasionally if it's chocolate milk. (Doesnt drink white milk that I've ever seen yet.) He is now more and more asking for milk in his cereal (still eats it dry more than he does with milk, but that's his option.) I just make sure he always has yogurt and/or OJ, spinach/other greens, cheese, and other things that have calcium each day.
I do wonder if all the problems we hear about milk indigestion and the "need" for alternate milks are due to our culture assuming babies/toddlers need to be drinking milk all day long. Do YOU drink milk all the time? Did people in the early days with a milk cow have their babies drink milk all the time? Even if they always drank milk, there were more people in the family to spread it around, so daily drinks of milk yes, but all the time? hardly.
Anyway, my husband does not like milk, and as an adult he understands that milk bothers his stomach. So he only puts enough milk to wet his cereal. Sometime drinks chocolate milk with lactaid tablets. All other dairy is fine for him, just not milk. I never cared for milk growing up, but dont have a problem with it. I always opted for my burgers without cheese, etc.
So when my son would NOT drink milk, it was disappointing, but not surprising, to me and I had to rearrange my way of thinking to making sure he was getting his calcium and Vit D without the shortcut of relying on milk. I will not push a food category on a child (ie, fish which also causes stomach upsets in my husband, peanut butter/nuts which my son only started to like/willingly eat at 5-6yo, and of course milk) because I feel that overdoing a food when a person is naturally resistant to it may cause an allergy or other reaction. Give them tastes, yes. But making them eat it everyday, no.
Sorry so long. Short answer: flavor it, and dont necessarily expect him to be drinking it everytime he needs drink. Water is more essential to our bodies - we cannot live without water but we can thrive without milk.