
I read the novel. Grayson did know it was Josh. According to the novel, he was just an asshole for inflicting trauma and fear. He mentioned he told Chase it was a dog, just so he could "force rut". Chase didnt like his ruts, so he always overdosed himself with meds, and when that one fateful night happened,Grayson knew it was Josh. But he said it was a dog instead. He's an asshole like that. It wad also from Side Story 1, when Chase asked if he really did sleep with a dog, or it was Josh. And Josh clarified it was with him

No, Grayson didn't know it was Josh, either the translation was bad or you misunderstood it.
The thing with the dog happened at his first rut (that's what Grayson said), that was when Chase still lived at home, and that has to be true, since Grayson and his dog being present wouldn't make sense any other way. He said he brought his dog, because there was one to spent the rut with, he would have gotten an omega instead of a dog in any other case. Grayson is crazy and his logic can be really problematic, but he is not evil.
When Josh slept with Chase, that was later, since he was hired by Chase and Chase didn't live with his family anymore at that point. No one knew who Chase slept with, Chase remembered that he slept wirth an omega, that was all. He searched for that omega for some time, but when he didn't find him he started to get some doubts.
Grayson was only told by Chase that he slept with an omega, but didn't know if that was true or not. So he didn't know it was Josh. And for him Josh was a beta, even if he had some serious doubts.
can someone tell me why is he afraid of dogs, is that what i think it is??? like the doctor said in kiss me liar "they'd even do it with a dog that appeared in front of them"