Just my two cents...
If you have any qualms about attending - then don't. I'm of the opinion that a wedding should be all about the people getting married. If you are NOT particularly close the the bride/groom and your presence might be any sort of distraction - I'd stay away, unless it is really super important TO HIM that you attend (which it doesn't sound like it is). Thank him for asking you then stick to more "family focused" events (holidays, etc.) until you are absolutely comfortable. (Then, again, I am not a big fan of attending other people's weddings in the first place - so I'd be looking for an "out" anyway.)
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (22+ yrs)
Dude: hetero poly male, live-in boyfriend (3+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe