2 mice a week for a 9 month old? That doesn't sound to me like anywhere near the correct volume of food for a fast growing young boa. A small rat every 7 days is a much larger volume of prey (depends too on how small the smalls are really though). Don't do huge jump ups in size however. Let him adjust slowly to the change from mice to rats and a much larger single prey item. You say you are feeding him a small rat once per week but only started this 5 days ago, so he's had what one feeding of rat?

We have a young male too. Unfortunately I lost all my growth records during a computer failure so I can't quote you exact weights at certain ages but I know he was way more than 370 grams which isn't even a pound by 9 months of age. This is our male at 10 months of age...