One thing that might help, suprisingly, is using a different type of bedding. What kind is that? Pine and cedar are bad for ratties because of the phenols in the wood, which are partly what makes the scent. It can damage their respiratory tract. That might help perk him up some.
Getting him new toys, even the cheapest you can find, is a good idea. For instance, take a toilet paper roll, fold one end shut, then put treats in, and fold the other end. Sometimes they figure it out really quick, sometimes they take a long time.