Why not put the groom in a vest to match the pants, or perhaps a tie?? A bow tie would be cute, depending on his style and comfort with that. He should definitely have a boutonniere of some sort too if he's not going to have any other accessories on him. At the end of the day, you have to remember he's the groom. The fact that he'll be beaming with the biggest smile, will definitely set him apart from his groomsmen. :-)