Your partner may have discovered that he particularly enjoys the physical sensation of one position and therefore tends to return to it.
That doesn’t automatically mean anything deeper.
It’s similar to other preferences in daily life.
Some people always sleep on one side.
Some people prefer one type of exercise.
Some people always choose the same seat at the restaurant.
Preference does not always require a psychological explanation.
Sometimes someone simply likes something more.
2. He May Feel More Confident in That Position
Confidence can play a major role in intimacy.
People do not always feel equally comfortable in every sexual situation.
Some positions involve more face-to-face exposure.
Others may make a person more aware of their body, facial expressions, or performance.
For someone who feels self-conscious, a particular position may feel easier because there is less pressure to maintain eye contact or constantly monitor their expression.
That doesn’t necessarily mean he lacks confidence in the relationship.