M.B.
My family is also dairy(casein free) and soy free (we all have low thyroid function and soy can contribute to that). Goat's milk protein is similar to cow's milk protein, thus isn't a good choice. And, lactose isn't casein free. Lactose is the sugar in milk and casein is the protein - and it's only one of the proteins. Whey is another cow's milk protein that generally must be avoided - they just test only for the casein.
The taste for the alternative milks can be developed and there's really no reason he needs to drink it straight. There is also a chocolate version of the Almond milk that my sons love. And, there are lots of other food sources of calcium.
Another alternative milk is Vance's Dari Free that is potato based and tastes very close to cow's milk. I made a large thermos of hot chocolate for one of my sons soccer games and kids (and adults) were coming back for seconds. Vance's is a powder that you mix with water, so it's also great for travel. It can be hard to find. They do have it at the specialty store at Flower Mound Pharmacy. Also, you order it online:
http://www.vancesfoods.com/darifree.htm
And, ideally, the whole family (at least at home) should go dairy free as it's very hard for a child to see others having things they can't - it will help tremendously with compliance. I bake with the alternative milks and you cannot tell the difference.