He should be on rat pups
Some say 10 to 15 percent of the weight till he gets over 500 grams
I would also look around for rat breeder as you are over paying at chain stores look on craigslist or check out a reptile show near you. Thry also sell online but if you only have 1 snake theb it might be hard to off set shipping cost but tgats a option slso.