i think i saw a previous comment say that it's around ch60 in the novel so that's a long ass time, especially with how Greyson mistook Dane's big boobs for that of a woman's and now he's checking off the women in the workplace one by one. but i also want spoilers so to anyone read the novel let us know
It happened exactly in chapter 70 of the novel. Grayson started suspecting that Dane might be an omega from around 60th chapter. Since he was curious why dane is sleeping around with ppl he just met as he is a beta so he don't need any reason to sleep with ppl he don't even know. Then he came to a conclusion that he might be an omega then as an Alpha can't hide the Pheromones
Oh and in ch 70 he just learned about dane being an omega. So he tried to date him in order to find of he actually is the fated one. He didn't have feelings for him at that time. After lots of ups and downs, Grayson realised that he love Dane and he is the only one for him. And this happened around the 100 chapter of the Novel (we have a long way to go)
No, its not casual fb. They only slept twice, one was when Dane in heat and they were trapped underground so there was no option for that. And the 2nd time when they trial dating and grayson was in rut but Grayson chose to take injection instead but the nurse messed up with the injection so again it was Dane who offered help to Grayson. Beside this they never slept until they become an official couple. There were just few kisses also initiated by dane.
Can't say for sure, but currently the manwha pace is fast if this continues like this it may happen around 30-35 chapter.

So on which ch he finds out his fated partner is dane?