They appear to be white cotton underwear briefs. There may be some Lycra in the fabric mix.
There may be a logo of some sort, white on white on his left hip but it is not readable.
My guess is that they are Jockmail with the branded elastic waistband folded inside to make them briefer. Not surprising he doesn't wish to discuss these briefs as it would mean admitting he folds he waistbands in.
You can actually see the waistband through the cotton fabric and the suspected logo is on the inside of the waistband and showing through.
https://www.ebay.co.uk/itm/Mens-Brie...EAAOSwzclewibx
Please tell me if you think I'm correct (or close).