Why does the audio switch constantly from quiet to loud when I’m on a phone call with someone who is using AirPod Pros?

When I’m on the phone with my boyfriend, he uses his AirPod Pros to talk to me. Recently, his audio has been going from normal to very quiet, barely audible, and then back to normal, over and over again. He tried forgetting and re-setting up the AirPods and we checked that it wasn’t either of our Wifi or data connections causing the issue. This has been ongoing for weeks, and he’s charged his AirPods, so battery is not the issue. Any suggestions?