I definitely suggest you borrow that scale and get a weight and the vet trip might be a good idea. I agree it's best to try first and get him eating before you freak him with anything else.

I use this this scale and really love it.

http://www.reptilebasics.com/My-Weig...l-p-16191.html

This particular supplier is amazing, great service, fast order filling and great shipping. Rich is my personal favorite to order snake supplies from.

You never know about size and age. We adopted a female ball python a couple of years back. She was confirmed to be approximately 18 months old but had been consistently underfed her whole life (the previous owners followed some very BAD pet store advice). When Kyna came in she was barely bigger than an average 6 month old ball python. With a proper feeding routine she very quickly caught up to female snakes her age and is in fact currently breeding this year with one of our males. Taken proper care of, these snakes are very resiliant.