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Everything should be on about the same level of formality. You picked a formal dress (and really really gorgeous by the way!), so his outfit should follow suit..pun totally intended.
I agree with everyone else that a tan suit is too informal. And beige tweed is too...70s? used car salesman? He should definitely wear a jacket - not just a vest. Plus I feel like the trend of grooms/GMs wearing vests only or pants with only suspenders and no jacket is beating a dead horse that has already been beaten for the last 4+ years.
1. That dress is gorgeous!!! (It's not that rustic/outdoors feeling to me, but I never did feel that every single detail has to be "on the same level" and it will look amazing anyway!)
2. I too vote for a dark grey suit, I think it would compliment both you and your general décor and theme nicely. I don't really care for tan/beige on anyone, and if you are having groomsmen it may be difficult for them to all find the right attire, where some of them might already own a dark grey suit...and will almost certainly find a use for one again if they wish to purchase one for your event.
3. I also think that your FI should be the one deciding what he wants to wear.