Mercury retrograde gets blamed for every stalled conversation, lost package, and ghosting incident that happens roughly three times a year. The actual mechanism: Mercury doesn't move backward. From Earth, its orbit occasionally makes it appear to reverse direction against the background stars — an optical effect of relative orbital speed, the same illusion you get watching a faster car overtake you on the highway.
Traditionally it's associated with communication, travel, and technology going sideways, which is a broad enough net that something will always seem to fit whenever it happens. That's confirmation bias doing the astrology's job for it.
He stopped texting because he stopped texting. People go quiet for reasons that have nothing to do with a planet's apparent path — interest fading, bad timing, someone else, plain inconsistency. Retrograde is real as an astronomical event and worth knowing about if you're into the mechanics of the sky. It is not an explanation service for other people's behavior, and treating it as one just delays the actual answer you're looking for.