I've given DS tea several times, he got into it over the winter as a warm drink, I use redbush which is naturally caffeine free and make it with a little hot water then top up with milk. It was just a phase here but he was much much younger than yours. I remember a thread here some times back and quite a few LOs were drinking tea
You could also add some milk powder to increase the calcium too.
Will he have those moist dried figs? I recently found some fig jam although that has sugar in but we just tried it for a change, what about either fig jam or pureed dried figs on toast etc or you can stir through custard or yoghurt if it makes those things more palatable for him??
Sultanas have lots of calcium too I think.
What about frozen yogurt?
Have you tried different cheeses? mozzarella is stringy/chewy a bit like cheese strings and really high in calcium, the blocks of it are even more like cheese string.
Have you tried milk shake or smoothy?
I recently found coconut milk powder which I can just stir through a cup of milk makes it taste very sweet and yummy even though there's no sugar.
Does he like apricots? kidney beans? Almonds?
Sorry can't answer on ice cream, my first thought is I wouldn't give it several times per week but I'm not in your position so who knows, I'd prob change my mind if I couldn't get anything else into him. What about whizzing sultanas or figs through yoghurt or milk pudding and freezing?? Don't know, just thinking there might be some reasonably easy recipe to make a healthier but tasty alternative?