We are going to a wedding in a couple of weeks time, with the dress code "cocktail attire".
The wedding is a church blessing in the late morning (they were legally married a few months ago for visa reasons), followed by a potluck lunch and BYOB in the church hall.
DH and DS (15) were planning to wear chinos and a checked shirt, but DH is now thinking that "cocktail attire" means he (and teen DS) needs to dress up, get a blazer etc.
I'm interpreting "cocktail attire" to be "nit suits, not jeans". I think - with the BYOB and potluck good in the village hall - that it's not going to be that formal, and no one will care if he doesn't have a blazer. I've checked with the bride that DS will be ok in trainers as he has size-14-and-still-growing feet, and these are all he has that fit and are cheap enough to replace when he grows out of them every few months.
The groom is my nephew. He normally dresses like Hagrid.