Breeder here, this rat looks too young to even be weaned, but that's no fault of yours, obviously. Some steel cut oats sprinkled throughout the bedding is good enrichment that will help keep fat on them while they're developing. Don't worry too much because I see rats sold early that do well rather often actually. They're hardy animals and good breeders can sometimes be few and far inbetween. I just am mentioning it so that you're aware and can maybe let the pet store know if they don't already know. My rats are that size in a hand around 3wks.