Am I giving my boyfriend an opportunity to cheat?

So my boyfriend has two devices and his text messages get sent to both. The other day it came up in conversation that he logged out of one to make it so his messages only go to one device. I asked him why he did that and he said it was annoying him that he was getting notifications on both. I personally think that’s a fake excuse and he did it for privacy reasons. Should I ask him if he wanted to change the passcode to the device he logged out of so he can log back in but just not tell me the passcode to it? Right now I have the passcode to the device but he’s a very private person and I think this is why he may have logged out even though I don’t know for sure. If I asked him to change the passcode and log back in (if he wants to do that) should I be worried that I am giving him an opportunity to cheat? Will he feel comfortable changing passcodes to everything after that? (Not the direction I’d want to go in, I would be doing this for him for a one time thing incase he wanted privacy)