One web site that might be of interest to your father is the USAF Pipe Band Alumni web site:

http://usafdrumcorps.us/pages/pb_index.html

I don't have my copy handy, but the Company of Military Historians produced a plate a number of years ago of the uniforms of the USAF Pipe Band, and a majority of the pipers depicted were wearing "mufti" Highland kit (i.e. civilian rig) instead of mess dress. If it were me, I would follow their lead and wear civvie rig, and your father could even choose to wear the "Mitchell" tartan, as it was the tartan worn by the USAF Pipe Band. It is also known as the "Galbraith/Hunter/Russell" tartan. My article gives a brief explanation as to its origin.

T.