My middle child (seven years old) hates dressing up. Well... sort of. He likes shopping for dressy clothing (slacks, vest, button-down shirt, tie) and has a respectable collection. He'll even occasionally wear it around the house when playing pretend, or dress all his stuffed animals in ties. However, when we actually need to wear somewhat dressy clothing to something, he "hates" the clothing, "looks ugly" in it, is "uncomfortable", and expresses these opinions repeatedly and at loud volume while getting dressed. He also sometimes outright refuses, and ends up doing passive resistance (just lying on the floor) while a parent stuffs him into the appropriate clothing.
I think that his dislike is somewhat rooted in the type of event that we'll be going to: it often requires a level of quiet, still behavior that is a challenge for him anyway. The clothes end up being a convenient scapegoat. However, this is just one of those unavoidable parts of life: there will be weddings, funerals, anniversaries, and other parties where dressing up helps show respect for the occasion.
I tried for a while to be very enthusiastic about how nice and grown-up he looked, but it didn't really make much of a difference. (It might have made him even shyer, since he doesn't like being noticed, but it's hard to tell.) He already gets to choose whatever he wants to wear. Are there other strategies that I can try to try to make this less of a flash point?