I think the number one thing you need to remember is that you should be unfailingly polite and respectful. A confrontational attitude will probably get you shot down immediately. On the other hand, you do not want to be timid either. Be confident when you lay out your reasons for wanting to wear the kilt, and assure him you are not doing it to make a scene or cause any problems. Threats are probably not a good idea as you may want to keep him on your side as a future reference. If he still says no, despite your best efforts, then wear a nice suit and have a great time anyway!