This place has large sizes, and a filter by boots available in particular sizes so you can see what might meet his preferances, and uniform requirements:
Find Large Men's Boots online from XLfeet.com. Check Huge Collections for Varities of Brands & Various types of men's size 15 Shoes online. Purchase at Best Price.
xlfeet.com
But... good luck. At a glance all are work boots, and I think most duty/670-1 boots stop at 13 or 15, only a handful go to even 16 it seems.
Have you tried calling Danner themselves? Most makers allow direct orders, and if that close, they might even have a non-stocking larger size or two they'll make him, can't hurt to ask.