They got him from a Petco around 2 years ago... And I weighed him today and he's a mere 144 grams and 18 inches. But not emaciated by any stretch- he just seems to be very small.
While I do not doubt you I seriously doubt this BP is 2 years old 18 inches and 144 grams.

This BP is likely couple of months old at the most.

As far as feeding if he is used to mice you can feed him one adult mouse once every 5 days until he catches up, than once well established if you want to switch him to rats, try rat pups!