You can certainly do it. Canned coconut milk is about 3 times as calorie-dense as whole milk, so adding 2 parts water to 1 part coconut milk should give an appoximation of whole cow's milk calorie-wise. I haven't tried it myself to see what it's like.
Nutrition Facts and Analysis for Nuts, coconut milk, canned (liquid expressed from grated meat and water)
Nutrition Facts and Analysis for Milk, whole, 3.25% milkfat
Is your son lactose intolerant? If not, I think milk can be a healthy part of his diet. Of course it'd be best to get milk from a local farm that is non-homogenized or maybe even raw if the farm is really good.