He wears T-shirts to weddings

DILEMMA

David and I have been dating since 2015. When we had just started going out, he used to wear casual outfits and that was fine with me, until he started showing up at weddings wearing T-shirts and sneakers. I have tried changing his wardrobe by balancing casual and formal attire but he has refused to wear suits. He says he feels comfortable in casual outfits. I am so embarrassed whenever we go to family and friends' functions that sometimes I prefer either going alone or staying home. What should I do?

Confused girlfriend