As a number of other members have stated, I viewed kilts as formal for years. I was raised with the view that kilts were expected to be worn to weddings, funerals, pipe band functions, Burns Nights, etc, & were just acceptable at a Highland games (if you were not competing). It took me ages to even risk a ghillie shirt. Now I've gotten comfortable with the casual look enough to be basically full-time kilted (extremely windy days still see me in denim, usually. No point answering THAT question for them!) However, I still think that nothing looks as sharp, neat, or respectable as full formal rig, & even those with no military training seem to aquire a military bearing when in a tank & PC.
Tell your Dad to look at pics of British or Commonwealth formal military dine-outs (a decidedly formal affair) or the same countries military funerals. That may just help to change his mind. Good luck!