Hi gsujillybean,
I don't think it will look silly at all! No one will be looking at him
!
As to renting a Tux, if he
wants to wear a Tux you can find one to
buy at Penney's or probably on eBay for about the same price as the rental!
If he doesn't want to wear a Tux, (my DH wouldn't be caught dead in one!) a dark suit will be fine.
BTW, I've learned over the years NOT to insist that my DH "conform". It doesn't work and just causes unneeded arguments
!
Hope you'll post pictures here when you get back,