Far from my area of expertise ordinarily, but because I'm getting married next year I have some some limited research into this topic.
Based on the time of the wedding, I believe evening dress will be appropriate. Depending on the formality of the event, you may do well in a tuxedo (two piece, black, with bow tie, no sash or cummerbund, black shoes but not wingtips, preferably Oxfords, white shirt) for a more formal event; or a lounge suit for a less formal event, which would be a two piece but with greater choice of colours for suit, shirt, shoes and tie (bow or regular).
Because it's summer, pick a light fabric for both suit and shirt. Whatever style sits best, go with that. Don't worry about slim vs regular fit. Pick something that looks good.
From memory, I know the advice to the groom for an evening wedding with bride wearing a classical wedding gown is to wear a tuxedo as I described - so I would guess similar guidelines apply to guests, especially close family.
But, honestly, if in doubt ask the bride. It's her day.