Here are a couple of links to info on a couple of models of window tint meters. If he gives you a dumb look like, "What the heck is that" you can explain it to him.
http://www.radiotronics.com/tintmeter/tintmeter.htm
http://www.solarstop.net/edtm/pdf/vp1165.pdf
By the way, if he doesn't know what one is I'd go to a different tinting shop.