The news of a Man of Steel 2 comes from Henry Cavill's new talent management firm, headed by Dwayne Johnson's own manager and ex-wife (they're still good friends) Dany Garcia.
“Henry has a big appetite,” Garcia told Newsweek. “We’ve been in a five-month period of time where he’s re-strategizing, acquiring property [for his production company Promethean], he’s filming [Justice League] now, he’s in development for the Superman standalone… he’s beginning to expand that world. It’s beautifully teed up. In a year from now, or two years from now, he’s going to be a force globally.”
WB first made it known when Justice League was announced that a new Superman standalone movie was being worked on, but no release date has been announced and nothing but rumors have been mentioned.
Per the above, we now see Dany Garcia confirms Henry Cavill will be in another standalone Superman movie and more as she says, "...he's beginning to expand that world."
Regarding those Man of Steel 2 rumors, George Miller was at one time said to be attached; Zack Snyder has both been said to be a part of Man of Steel 2 and not having anything do do with it; Brainiac and Supergirl are rumored; it's also said Man of Steel 2 will be releasd in 2020.
Henry Cavill next stars as Superman in Justice League, which has a November 17, 2017 release.