Hi @geke
Colas are the common softdrinks that contains phosphoric acid, the reason for that maybe as a stabilizer for that food color they use as not to curdle but I am not sure, possibly for flavor too. One thing to consider is that it is very high in phosphorus. You can check the ingredients of other drinks if it does contain such ingredient.
There are baking sodas that do not contain alum so maybe you can use that if you like as alum also is absorbed by the skin.
I just take calcium supplement myself but sometimes it makes my stools hard so I taper it off a bit.
Your father must be managed by a specialist for his parathyroid issues, it is a delicate condition that will affect his bone health.

Thank you @geke , I appreciate your support.