Backyard wedding guest attire help!

So FH and I are supposed to go to a wedding late afternoon today, but we have no idea what to wear! This would be a casual backyard wedding (invitation was sent on FB messenger). Ceremony would be at a court house, but we can't attend as both of us would still be at work. Reception will follow at the couple's house, starting at 5 pm. Also, it is hot and humid in the area right now. I would probably be okay with a simple dress, but what about FH? I thought about having him wear jeans, a button up shirt and a blazer, but jeans may not be proper. But if he wears a suit, it would be too hot and he worries that he will be overdressed. What to do? Ideas please!