Well, my instinct would be brown, or even tweed, suits, but the groom wants tuxedos. Would he consider dark grey suits? That would be more comparable in formality to what the ladies are wearing.
If you do Tuxedos, the vest (or cummerbund, preferably, given the style if vest normally available to rent) should match the tuxedo. So, black or grey, depending on the colour of the tux. With black, a black self tie bow tie. With grey, you can do pretty much any colour of tie. I’m inclined toward traditional wedding ties. Which means a silver/grey/black formal pattern like glen check, or Houndstooth. You could do brown, or rose. Pink and grey are a great combination. Navy ties will also be very nice with grey.
If he really, really wants contrasting vests, I would do green, buff, or brown, wool vests, but satin ones. I dint think the satin vests will work well with the country theme. Try mytuxedo.co.UK for vests – you can probably still then in eBay after. This is borrowing a bit from morning dress, and, while I dint think it’s the way to go with a Tuxedo at all, it is definitely only good during day light. If it’s after dark, or after 6, stick to classic black tie.
The groom should read blacktieguide.com before making a final decision.
Traditionally, blues and greys are city colours, and browns and greens are country colours, as they reflect the landscape. Black is reserved for the evening, for the most part. That’s why my first thought was browns. You don’t have to stick to that, but this way you can decide whether you want to stick to those traditions, or not.