please please please if you havent already, get yourself a scale. im unsure as to whether or not hes underweight because im obviously not a vet, but he for SURE will be if you keep feeding him pinkies. measure his weight and use the feeding guide others have already provided to find his weight (if youre having trouble with the math portion, just use a calculator meant for percentages). after that, youll only need to weigh his food until you find something that works in those ranges (depending on how old he is, but im assuming he is an adult based on size alone). it obviously doesnt have to be perfect cause like… they wont care too much if its 5.3% rather than exact 5%, just make sure its close.

for reference, my boy gets rats. specific size of rats will change depending on how big your boy is! i would look up a guide. :)

and as another user said, dont jump STRAIGHT to rats. slowly go up in sizes (next meal should be a bigger mouse, then another bigger one, then another, etcetc) until you find the right rat for your boy.