He knows on which side his bread is buttered.

The proverb describes someone who recognizes which person or arrangement serves their interests and acts with that advantage in mind.

It can suggest practical good sense, but it may also hint that the person’s loyalty or behavior is guided more by personal benefit than by principle.

Use it when someone is careful not to alienate a person they depend on, or chooses a course that protects their own interests. The tone can be approving or mildly critical, depending on whether that calculation seems sensible or self-serving.