I think that the concern with jumping from hopper mice to small rats is not as much of a size issue as it is a species issue. Many ball pythons take a long time to transition from mice to rats, and starting this process now is likely not the best idea as the animal is underfed and wouldn’t be getting as much food during the transition period. What the others are saying is that it’s more important to get him to a healthy weight right now and transition to rats after he has gotten accustomed to the size of an adult rat.
Someone more experienced than me is totally welcome to correct me, this is just my take on it!