Re: Tantrums in the grocery store
at 21 months he should just be told to sit in the cart and be quiet in a firm and straight tone without fussing. We have been talking about this type of stuff in church. But reward should maybe be another idea or walking by helping get/mark off stuff/push cart (like my little one does some) or if you babywear try that.
although it is hard if the tantrum is at end like my oldest did when she saw the toys at the check out and wanted it.... I just wanted to get out of there and not go back but it was a clothing store not grocery though.