It is rude to not respect the dress code at events. In some it might get you barred from entering.

This happened to me once when I was in my early 20s. It was the first wedding I ever attended, and it was a pretty formal affair and in a foreign country. My boyfriend was the cousin of the groom. So we get there and I have brought a very nice dress and my bf, is wearing a shirt and jeans. So I called his cousin the and asked for the brides number. I called the bride and I told her my bf decided not to wear a tux, and that I knew he was in the family foto, and told her if it would bother her, to talk to her fiance and have him dress down my bf.

And he did, later that night he called and gave my bf his opinion. And we rented a tux and went to the wedding and broke up when we got back home.

But what an asshole. Honestly. Think about it. You are close enough to the groom to be in the foto. His mom is basically your dads best friend. And you intend to be the only person there in jeans and a shirt. Fuck that. - I am completely fine with admitting I threw him under the bus. But if that had been my wedding, I'd have wanted to avoid having him in the foto dressed like that, and then I'd have to make a scene and demands on my wedding day which would have soured the mood.

So, I encourage you. Think about contacting the couple. Telling them your bf is a child and you have done what you can. But here is a heads up and to contact him directly if want to dress him down so to speak.

Ps.

You should have told him that you would be embarressed when he asked. Why would you lie? He is making his bed poorly, why not let him know? He is an adult and his fragile little mind can handle being told when he is a jackass.