Don't stress over the milk. Calcium is in many foods other than milk. My daughter doesn't like milk. I did all the same things you are doing when I was weaning her off breastmilk and onto milk. She's 19mo now and still doesn't like milk.
We don't need to rely on milk to get Calcium (or fat for babies). Here's my recommendation - stop putting your engery into getting your son to drink milk and start looking at adding high Calicum foods to his diet. Your son will need 3 portions of Calicum rich food throughout the day.
My daughter loves the yogurt so I get YoBaby yogurt b/c it's high in fat (all the adult yogurt is low fat). A slice of cheese or a cheese stick, cottage cheese, etc, will all work great (and cheese has plenty of fat in it).
Since my daughter isn't drinking milk, I give her water and watered-down juice throughout the day. She actually likes V-8 juice, which is weird to me, but I give that to her some too. She loves OJ too. My daughter is just now getting a little more interested in stuff like chocolate milk, but still won't drink plain milk from a sippy cup. She might come around some day, but I didn't want to fight her on the issue when there are so many other options out there. I've heard that some kids instinctively avoid foods that they are allergic to and I didn't want to risk that being the case.
Here's a list of Calicum rich foods:
Yogurt (1 cup has more calicum and fat than milk)
Cheese (Cottage Cheese, Mozzarella, Swiss, Parmesan, etc)
Broccoli, Peas, Spinach, Greens (Kale, Collard, etc)
Salmon
Beans (White, Baked, Black, Navy, etc)
Plus, there's a ton of Calcium-fortified foods out there(some breakfast cereal, orange juice, bread)
Good luck!