Technically, you are not "using" his textures. You just gave a surface in a bsp file the texture attribute to use the texture available at pball/textures/whatever. I doubt he has a copyright on that filename because at least here in germany, a filename doesn't have enought creational value. As long as you don't distribute the texture itself, he probably can't do anything about it. There are many cases in the software world where this way is used to avoid licensing issues, for example Audacity doesn't (or at least used to not) distribute the LAME MP3 codec, they just link to it to their main website and people have to download that separately. If he wants people stop using his textures, he can only initiale legal steps against server admins that distribute his textures, but then again he published them so admins can distribute them, so I doubt that would lead to any success.
Still, in communities like this it would be nice to respect other people's work, and if he asks nicely you should probably fulfill his wish.