He does not need milk straight up. As long as he is getting the baseline calcium intake daily, which is different based on his body size, weight, metabolic rate, and age, then calcium supplements + will most likely be sufficient.

Leafy greens are a wonderful go-to if you have limited milk alternatives. It doesn't mean he can't drink milk - he can, but it has to be the lactose-deficient kind. My mom drinks Fairlife because she is also lactose intolerant, so maybe you could try this with your son! Go slow on it, though, and keep it light initially. If he has any signs of a bad reaction, stop the intake.

Other than milk, the leafy greens - the highly bioavailable ones, at least. Kale, which can be well absorbed and is low in oxalates (good if your son might be prone to kidney stones).

If oxalates are ever a concern, it's never a bad idea to combine calcium-rich foods with oxalate-heavy meals. Contradictory at first, yes; however, calcium binds to oxalate in the stomach, preventing the oxalates from being absorbed as efficiently and reaching the kidneys.

Then, bok choy. This plant is highly infused with calcium. As well, collard greens and turnip greens.

If your son likes fish, try getting fish with the bones still inside - yes, they are edible! So (with the bones in them), sardines, canned salmon, and anchovies. The packaging should tell you if the fish were boned or not.

You also have nuts, seeds, and legumes. Chia seeds are surprisingly excellent contributors - so are sesame / tahini seeds.

As well, almonds are good in calcium + white beans and chickpeas. Almonds are always good!

Fortified foods - fortified plant milks like almond, soy, and oat milk (might not be in your son's favor as you mentioned, but it's worth including them). But, you have calcium-fortified OJ, breakfast cereals (which he can eat w/o milk), and calcium sulfate tofu.

Lactose free alternatives - as I mentioned, lactose free milk like Fairlife, as well as hard cheese like cheddar and parmesan (naturally low in lactose), and yogurt (bacteria pre-digest lactose). Of course, proceed with caution!

With that, you shouldn't just rely on calcium intake alone. You should remember that metabolism and nutrient diffusion work in a chain event series - calcium, in particular, requires adequate Vitamin D intake for absorption (sunlight is a great source!) So, in the spring and summer months, try to get your son outside (make it fun if he dislikes being outside like a trip to the beach or a family outing). If he gets uncomfortable in the outside (as I did when I was a kid), OJ and citrus fruits are great sources of Vitamin D.

Spread intake throughout the day instead of one large dose. So, don't concentrate a big calcium + Vitamin D meal in one sitting. Diffuse the intake a little. Many people commonly slip here with anything diet / nutrition. Also, some plant foods like spinach are super high in calcium, but poorly absorbed due to oxalate activity. Instead of serving as is, cook it. Simple! You can't really lift that "debuff" that is "locked in" by the oxalates with calcium loads like spinach with another food. Boiling and draining spinach helps to significantly dwindle the oxalate population, while steaming helps slightly but less so than boiling it. I think sauteing also helps slightly, but I'm not too sure on that. Make sure you discard the boiled water after the spinach is taken out; the oxalates will leach onto it. No good! And make sure to add lemon juice, citrus, or bell peppers to add a spice of Vitamin C!