I've never had to worry about any children getting married Rosie but I think I'd go for something unconventional outfit-wise but then I suppose it depends on what style of wedding they have - I think the only rule in general is 'don't outshine the bride!' Get your Graham in a kilt though!
Definitely go with the kilt for Graham. I had two outfits, both from Myer. One for the first night party and for the wedding, trousers with a sort of sheer net top over a cami heavily embroidered with bright flowers. Steven is 53, so I did not go with the Mother of the Groom outfit, but wented to be modern.