I agree with Beachlion. Try some cotton thongs as cotton absorbs sweat. Calvin Klein has some cotton thongs.
I am also a big guy, and I wear L and XL. If the fabric is elastic, I go for L thongs. If not, I go for XL thongs. Anyway, I think your man has to try out different brands and see which one suits him. My personal favourite is Gregg Homme.