When Mr. Buckley (Blake's teacher) comes out of the shower at the guys' house, he apologizes to Blake for using an old scruffy toothbrush that he found in the bathroom, and Blake immediately answers "Oh, no problem, I'll let Ders know." What Blake's reply means, is that it was Ders' toothbrush, and that Blake would let Ders know that someone has used his toothbrush.
Blake's reply also implies that out of the three guys (Adam, Blake and Ders), only Ders cares somewhat about dental hygiene to actually own and use a toothbrush, even though he has apparently been using the same worn toothbrush for years. In other words, it's implied that Adam and Blake never brush their teeth and therefore don't have toothbrushes.