thing is he is ok about drinking milk but just wants it in a bottle
Would he take a straw in a 'cool' boys cup or a sport bottle instead of a baby bottle?
You can add ground almonds (and milk powder) to things like pancakes and muffins if you ever make those. In fact you can just switch all the flour to ground almonds to make pancake mix. Evaporated milk is high calcium too (from memory it's about double regular milk), in case you didn't know, don't know if you could do something with that so that smaller portions can still equal decent calcium level.
I did a sort of icecream a couple of times with frozen fruit and regular milk in the whizzer, it is softer than proper ice cream like it's been left out to melt a bit but still thick enough to eat with a spoon and it's cold and tasty without the cost of buying an ice cream maker. You could prob do a couple of frozen ice cubes of milk, a handful of frozen fruit and then enough milk (or evaporated) to loosen it, the non-frozen milk goes thick and cold because of the frozen fruit.