I know I'm going to see a lot of "why would Chris call Ramon and Helena and not Buck" and I think it's literally as simple as the fact that Buck is inextricably linked to Eddie.
"But Chris called an uber and ran to Buck before" yes, but this isn't 'dad started dating and I'm mad' this is some serious hurt and confusion that Christopher is going through, and I don't doubt that he knew if he called Buck, and as much as Buck would have been there to support him, Chris wanted to get away from Eddie and get space, and Buck was never going to be able to give him that. Buck would never be able to separate himself from Eddie in this.
Yes obviously Ramon and Helena are Eddie's parents, but they've proved on more than one occasion that they will put Christopher's needs above Eddie if it comes down to it. And Christopher needed space, he needed time to deal with his own feelings, something he knew his grandparents would be able to give him. And I think maybe he knew that Eddie would give him that if he pushed hard enough. Plus he's 13, he probably feels betrayed, part of him probably wants to make Eddie feel even worse (remember what 13 was like?).
Also, Christopher can rely on his grandparents in this as a support, Eddie needs Buck. In this situation Buck can't play mediator, he needs to be Eddie's support through this (even if Eddie does decide to push him away). I'm glad they gave Christopher some agency here, especially as someone so young, to decide what he thinks he needs. And Eddie's response is perfect, "five minutes or five months", he supports his decision, even if it's going to break him to do it.