Let's add that even though milk contains a lot of calcium, it's not very helpful in getting it to your bones. Proteins in milk act as acid and to buffer that, your body releases calcium from your bones. You gain some but lose more.
An even better source for calcium is, believe it or not, bones. They have way more calcium than leafy vegetables, and for us carnivores they're definitely the better option because most of us can't fully digest plants, and thus don't get any benefit from them besides fiber.
Not sure if that'll benefit him though, might be harder for him to get the calcium into his bones no matter where he gets it from.
We get way more from plants than just fiber. Proteins, fats, sacharids, vitamins, antioxidants, minerals including calcium. Our ability to digest vast diversity of plants and their different parts is rather astonishing. Maybe you mean our inability to digest grass (and also cellulose in general). We're really not good at that, unlike e.g. cows or sheep, who have specialised digestive tracts with symbiotic bacteria that help them, so they can feed on grass alone.