The rule that Scotsmen should not wear tartan south of the border has been relaxed over the last few decades. Nowadays it is most acceptable to wear tartan, but wearing a kilt when there is no Scottish (or Gaelic) connection might be seen as a rather pretentious gesture.
The groom may wear his own tartan - modern, ancient or dress.
The kilt is accompanied by either a short tweed 'day' jacket and matching tweed waistcoat worn with a plain shirt and tie, or a Kenmore velvet jacket and jabot.
Kilt hose (never white or cream), garter flashes and Sgian Dubh (the dagger) are also worn.
The type of jacket dictates the type of sporran to be worn: a leather day sporran sits with the tweed jacket and a dress sporran (probably decorated with fur) accompanies the velvet jacket.
The other option is to wear tartan trews with a morning coat.