If he was on medium rat there is no wonder he is not eating, medium rats are too big for a male, ideally you want to feed a small rat every 7 to 10 days.

When fed to much BP will fast to catch up and considering his size it could last a few months or even a year, do not offer alternative prey you may be stuck with, simply make sure your husbandry is on point and most importantly be PATIENT.

As for the weight loss not knowing how you weight your snake it may or may not be explainable, however if you are concerned have a fecal done.