Dogs, and birds, and cats oh my

Actually his brother lied to Chase fucking a dog. In fact in his first heat he did it with Joshua.

Nope, in his first heat the dog was present, although nothing happened.
Josh was much later.

Wth! r u Serious?!
I think his brother is just lying and making trash jokes

He does make those trash jokes, but when Chase had his first rut he panicked.
There was no one there to help out. The pheromone shit is really serious in this universe, their dad fried his brain because of that and became emotionless. So he probably heard or read this stuff with dogs (I heard that in another story on another continent some stuff like this comes up too, and that doctor most likely also talks about it), so he got his dog. But luckily Chase passed out and they used an injection, and from another story I guess that their dad arrived ( their dad actually used injections in the past).
Problem is that Grayson later lied and used Chase's fear to make him go release pheromones. It actually should be explained in a few chapters. And Chase says something about Grayson's problems at the end of the mains story.
Grayson isn't actually as bad as most think, but he is a born manifested and that's why has some serious problems. He really wants to help Chase, but his logic and problems with understanding emotions makes the way he tries to help others sometimes really damaging as a side effect. The only thing he sees is the result that Chase releases pheromones, but he is blind to what it did to Chase. And it's not just Chase, it's big problem in general.
Chase really suffered because of the misguided help of Grayson and his twinster ( when they were children she was always involved) and the lack of affection of his parents, because of Ashley being paranoid.