I am so sorry you and your daughter must be around such ignorance. The only way that he would truly understand what you are going through is if he had a diabetic child of his own.
I think the best way to deal with it would be to try to shut down the conversation next time he brings it up. Have a polite but firm comment ready that basically says to him that the subject is not open for discussion. If the child is a problem I would handle it through the school or the dance studio.
I wish you the best.