You need to feed what works mice or rats grams per gram is all the same the difference is when they become adults you are looking at feeding one prey vs multiple.
The problem with switching is that it can take time and tough love and at this weight it's not something you can afford especially since the animal was underfed, I would recommend to keep him on mice until he reaches at least 100 to 150 grams and then switch to rats, by that time a little tough love if done right when switching will not be a big deal.