Any words of wisdom from the forum on how you can broach the subject without letting your spouse know you are spying on them?
Is your question actually how to let your spouse know without revealing your method of snooping? I appreciate that if you let him know that you have keylogger on his device, he will just use another device.
Tell him you know, without telling him how. You don't need to prove that you know. He knows that he does it; you will just be telling him that you know he does it and that you want it to stop. Don't let it turn into an interrogation of you. It is he who is in the dock.