I'm guessing that you're referring to the logo on his hat...is that something like the quiksilver logo? (is quiksilver a clothing line?)
Actually, I think in the episode Primeape Goes Bananas, the origin of his hat is revealed...it was only one of a hundred, and it was a promotional hat for the pokemon league. Ash sent in a great deal of postcards and got the hat, and the logo is that of the pokemon league.
Besides, the show wouldn't be allowed to have any real brands of clothing or corporate logos in the show anyway.