Gerbils are very addictive to BP some will never switch back.

In the end switching prey type because an animal is fasting is never a good idea, the best is to wait it out and be consistent with the prey type being offered. Winter fasts with an animal that age are normal and patience is key.

I would recommend no to offer food until he readily accept what he used to eat in the past, tough love will be key in switching him back.