Gerbils can suffer tail loss from excessive handling or too much stress. Have you ever tried to catch him and accidently ended up holding onto the tip of his tail? I know how easy it is to do that from experience with Tia (rat) so I'm not blaming you for causing it. The skin on the tail is very loose and it is in the gerbil's nature to shed the tail, like a lizard loses its tail to distract predators. I don't know if he that's the cause, but it could be.Try and get him to a vet as soon as they re-open. I really hope he's OK. ((hugs)) to you.