I would try and get him onto fuzzy rats (perfect prey has fuzzys between 10-19 grams, and your snake should be able to handle around 13-15 grams easily). Mice are ok - but he still needs prey that is around 10% of his total body weight until hes older. Rats are more nutritious and it will be easier to feed once hes an adult. (1 rat versus 3-4 mice).
As long as he is healthy and not underweight now - I'm guessing if he is as old as you say, he was probably thin from being at the pet store - then you can try and switch him now. it's easier when they are young and always ready to eat.
Also, if your worried about size of prey, I had good luck calling perfect prey on the phone and asking if there was any way to get the smaller end of the weight rage when mine out grew weaned rats but could not yet handle the high end of the small rats. They are great and offer smaller quantities of prey if needed.