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,