Progressing to a larger prey size - when?
It all depends on the size of the snake. My oldest ball python is between 2-3 years old and takes small rats with no problems and last week he took a medium sized rat. The basic rule of thumb is that they can eat the same size prey as the widest part of thier body. Jojo is about 30" long. I say get a small rat and see what happens odds are is that he will be able to take it down. They wont try to eat anything that is too big. Its absolutely amazing how much these guys can eat.
Also feeding a juvenile rat instead of a baby rat offers more calcium. And when you start feeding rats Icculus will start growing like a weed.
Happy snake keeping.