If it is winter get a heater to run in the room over night. You may need to move the water bottle so that the animal can get to it easily or even hand feed water with a pipette. Isolate any other animals that you have in separate tanks but don't remove the animals cage mate as this will be upsetting and he will already have been exposed to the flu by the time it shows. We lost one gerbil, but the others all survived although I was reduced to giving injections to Biscuit myself over the weekend and my husband sat up all night feeding her water. She recovered and lived to a ripe old age.
Elderly gerbils can get strokes.
The animal will loose use of part of it's body and fall over or have problems moving. They have the most amazing ability to get over strokes so make sure that your pet can reach food and water and keep him warm. Again you should seek advice from a vet as strokes and middle ear problems sometimes have similar symptoms. Birth Defects Gerbils are amazingly resilient to inbreeding but there has been rather a lot of it done particularly for the new fancy varieties. This and other factors inevitably result in some birth defects.