If you find that you tell him things you haven't told your partner, or that his opinion has started to matter more than anyone else's, that's not a moral failure. It is, though, a signal worth taking seriously — usually less about him and more about what's gone unsaid in your primary relationship.
How do I tell whether a friendship has shifted?
Watch the small mechanics rather than the feelings. Has the contact moved to hours when other people are asleep? Do compliments land differently than they used to? Do you dress with him in mind, or feel a flicker of disappointment when he brings his partner along? None of these alone means anything, but a cluster of them usually means the friendship has changed shape, and the kind thing — to everyone — is to name it early rather than pretend you haven't noticed.
Should I tell my partner about a close male friend?
Yes, and preferably as a normal fact rather than a confession. Mention him the way you'd mention any other friend: what you did, what you talked about, what made you laugh. If your partner reacts with real distress, that's a conversation about reassurance and boundaries rather than a reason to hide the friendship — and if it keeps causing conflict you can't resolve between you, a couples counsellor is a much better referee than a group chat.
