It could be that he has an intolerance of some degree. I would talk to your ped about other calcium and vitamin D sources.
Have you tried almond milk? Coconut milk?
I take a chocolate calcium supplement (with vitamin D) and my DC have taken tums for calcium in the past also.
I didn’t know much about supplementing for iron, but will your DC eat spinach (raw for salads or cooked with lemon?) and meat (beef/chicken/turkey)?
Hang in there, mama. Your doing a great job.