Panache has given good advice, here. The full PC sounds like WAY too much for the occasion. Ditch the vest, use your buttons on the jacket and wear a nice, wide dress belt. Have him wear a regular white dress shirt and a regular necktie in either black or a solid tie in a color from his kilt, instead of a bowtie.

Get more buttons (smaller ones) and finish the vest before the lad gets married.