Rambling on why I'm OK with Callum not finding out yet
Fundamentally, I think that having Callum find out after Ben's managed to turn the corner for himself instead of being the person who saves him (yet again) opens up much more interesting story space for them both as individuals and as a couple.
Because they've been down this road before. Not for the same reason but Callum has supported Ben over and over again. And it's been lovely! But it's not a sustainable position, because the moment Callum's support is withdrawn or he doesn't get it quite right (and he can't always be perfectly 100% there for Ben because he is a flawed human being like everyone else) then Ben collapses into a self-destructive spiral - he withdraws Jags' protection, or picks up a random guy and then steals his car, or starts beating the shit out of random homophobes. Callum has done the best he's capable of and it's not enough because Ben needs professional help to deal with his shit, and he won't get it so long as Callum is there to pick up the pieces and make him feel better.
So, for Ben as a character I really want him to find his way to wanting to live and be better and be happy for his own sake. I'm also really enjoying the development of his other relationships, which have all (apart from Phil) taken a backseat while he's been with Callum.
In terms of Callum as a character, I'm always excited to see him written as a real person with flaws who can't read minds, and for me how he's acting right now makes sense. I think he's forgiven Ben a lot over the past few years because he thought that if (a) he loved Ben enough, and (b) he could get Ben away from Phil then Ben would be the good man Callum believes he is deep-down and it would all be worth it. But it turns out to be more complicated than that (for multiple reasons that Callum's not fully aware of) and now Callum's dealing with the built-up resentment from previous rows and also has lost faith that things will ever be different. It's not just the cheating - Callum was really (and legitimately, IMO) upset by Ben committing those attacks. And while I think he could have been more understanding of where Ben was coming from, I think Callum was bang-on when he suggested that Ben get counselling. Callum can't be Ben's therapist (and neither can Kathy, or Jay, or anyone else that loves Ben and is trying to support him). If Callum thought he could save Ben I firmly believe he'd be there right now, regardless of whether or not he knew the truth. But he doesn't, and I don't think that he's wrong.
(Also lbr I love angst and suffering and I don't want Callum to miss out on his share! The guilt when he finally finds out will be exquisite.)
So if Ben does manage to find his way through this without Callum's help, whether that's reporting Lewis to the police or speaking to a counsellor, I think that opens up really interesting new ground for them to navigate as a couple once Callum does find out, and gives them a much sturdier ground to build their relationship on going forward. Much as I'd love Callum to be involved right now, I think ultimately it would prove to be a short-cut that would avoid some really crucial character growth for Ben, and in the long run that's going to be essential for their relationship.