First, I'd suggest contacting a few of your clients from the past 1+ years that you've already worked with. Get their feedback. To me, I think that would be
a valuable bit of information to show him that clients are not put off by the kilt.

As far as my job, I don't really work with outside customers in person but those who have come to the office and seen me in a kilt have not been negative. I work with commercial construction sales representatives. My inside customers (co-workers) give me ribbing on occasion, but it's a non-issue. It helps that the owner of the company approves, so no one else's opinion matters.