How horrific. I'm so sad for him & for you,too.
You must be strong for him.
I had to endure a very similar situation & as the revelations unravelled I had to fight to stay strong & stoic for them.
My heart was utterly broken for them & yes, I shed tears but, I had to swallow down my own rising panic & disgust at the perpetrator, my rage,etc.in front of the people confiding in me.
I have managed to not show the chaos I was feeling as in our situation normality & me showing nothing had changed & that I wasn't going to do something out of control was needed for the people suffering.
In our situation,police were informed & hopefully a satisfactory outcome shall come soon to allow some kind of closure.
In the meantime, always being available to talk when they want to, no pressuring them into discussing it, staying (outwardly) calm, lots of hugs & reassurance that nothing has changed within your own relationship & life with them & that you still love them as much,if not more have all helped. We also, found just doing mundane family stuff helped strengthen the bonds & demonstrated how nothing had or would change despite these horrible revelations.
It will be hard op,but, you will get there. If you love someone enough then it's amazing what you can cope with.
I wish you lots of love & luck.