What could have been the reason(s), why he’s been treating by his brothers like that? That was annoying, to be honest. Do you have suspicions or theory(s) about his brothers. For me: maybe they were his half-brother (just my suspicion).
Spoilers:
No, they're full brothers. They didn't grow up feeling love. Their alpha father didn't care for them and an incident with their omega dad, made their alpha dad treat them like soldiers and he rarely made them see their other dad, the only one who showed them love. Grayson has been dominant alpha since birth. He didn't get to transition and live a normal life, he always just was. In their universe, dominant alphas who don't release often, get brain damage. Grayson had violet eyes since birth, so he's always been cray cray. Grayson has been told by their omega dad that helping others is good. Unfortunately, his idea of helping is a little bit twisted as he doesn't have empathy and has no feelings. Him helping Chase since they were little, traumatized the fuck out of Chase. He actually feels love for Chase and is concerned about him. Chase is suicidal, doesn't release pheromones. To get Chase to release pheromones, he scares him about the dog. It's unfortunate, but it's what it is. It's all sad
Yeah he has a really messed up family and past. There's a reason he doesn't like omegas or being around them. His family made him do a ton of stuff he didn't like, and there's a lot about omegaverse-related stuff that he doesn't know about because he wasn't properly educated. We will see more about his past and his family in the near future