Hi there, Hislander, and welcome to X Marks from the bottom end of Africa.
Since nobody seems to have chipped in with any advice for your wedding, let me put my boot in (having worn kilts in the army, but never to a wedding!).
Since the occasion is informal, you need not get excited about anything fancy. All you need is a shirt – preferably in a colour that does not clash with the kilt, and perhaps in a colour that matches it. (You could also pick a colour that highlights the kilt without matching it.)
That is to say, it can be white, black, or any shade in between.
It can be what you folk across the Atlantic call a dress shirt, or it can be as casual as a golf shirt or a T-shirt, all depending on what your bride feels is smart enough.
You can wear a tie, or not.
If you wear a jacket, make sure it is short enough not to hinder your access to the sporran. (I suggest that you only wear one if the weather demands it.)
Will your little boy be carrying the rings?
Regards,
Mike