Hi,
Not sure what you mean by secret.
However if he is using the iMessages on Several devices with linked iPhone Number and Apple ID then it is likely that it appears on all the devices.
Even if he turns off the iPhone Number on the Mac for instance the link to the Apple ID means he still sees it there.
8:36 pm Monday; November 6, 2017
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)