I would decide what you are going to feed him first, rats or mice, by what is readily available. I order my frozen rats online in bulk.
A little story...
A few months back I was at PetSmart getting some dog food when I came across their Ball Python enclosure. There was one lonely Normal BP left. As I looked at her (confirmed she was a she) I noticed she had a bad shed and seemed a little small. The temps and humidity were way wrong (20% and around 75 degrees-enclosure was just inside the front door about 20 feet and in the middle of Winter) in the enclosure. SO I took it upon myself to "rescue" this BP from the wicked PetSmart keepers. When I saw that she was 3 months old, I immediately knew she was in danger if I didn't buy her. So I did, took her home, weighed her and put her in quarantine. She, at only 3 months weighed a measly 100g. She was being fed ONE mouse fuzzy a week at PS. I kept her on the one mouse fuzzy for a couple of weeks, then moved her to two mouse fuzzies a week for about a month. Last month, I switched her to rat fuzzies and she is now onto rat pups...and she is thriving! She is currently 5 months old and weighs a little over 250g. She has come along so well and am glad I was able to rescue her from her PetSmart hell.
Long story short, I do use the 10%-15% rule right now in my two young-ins' and they are thriving very, very well.