I will recommend to you what was recommended for me.
If you are able to do so. Go to Online Breeders like
"Layne Labs" or "Perfect Prey" they have any size you will need, and you can buy in bulk.
And as far as size transitions go...
For awhile when Charlie was 250-350grams i fed her two
15g Rat Fuzzies before i bumped her up to one 35g Weaned rat (i didnt think she was quite ready yet, weaned rats can seem massive compared to your tiny snake). Just be sure he is eating enough but not to much!
And soon he will be Pounding Weaned rats like Popcorn!