The burgundy and brightly colored shirt is a concern. It is one that really will mess up the whole thing. I have a couple different ideas. First, you could offer him the option of a "wardrobe change." For the wedding itself, keep it traditional, white or blue shirt. Then for the reception, allow him to do the burgundy shirt / tieless thing. I think he would probably be OK with that.
And I think that you have the right idea. Women have their ways to make a man feel good. Take him shopping, and splurge a little bit on a shirt. Make it your treat. I think most guys are afraid of looking like they 'care,' or, even worse, looking like they care about how they dress and get called out on it. I think that there is more than enough room for some middle ground. Take the suit with you as well, so that he can actually see what he looks like. You know how women pick out a blouse, hold it up against them, try it on with something that is close to what they would wear with it, see how it works with their complexion, etc.? Men don't do that. Heck, I don't do that. If I see a pattern that I like, in colors I like, I buy the shirt. And sometimes... it sits in my closet because I realize I own NOTHING that it works with. That is what men do. So bring the suit, go to a nice men's store, and buy him something you both agree on. I think he will respect your veto power, the same way you will respect his veto power. You have to nudge guys into this sort of thing, not go whole hog.