I may be the only one who thinks this, but wear a tux. Yes a nice one is expensive, but you should be able to find something in the 500-600 range. Its your wedding not a job interview, and its in the evening. I would be more flexible if it was day time, but a formal evening event, that you are basically the host of, you should be in a tux. just my 2 cents
I think you have a valid point, but as the bride is not going formal a dressed-up suit is a good match. If he were to wear a blue blazer with gold buttons, cream pants and a repp tie I would be concerned.