Florida king snake not eating

Question
Hello.
I currently have a 2 year old male albino Florida king snake. and for the past 6 weeks he has refused his food. I feed him on a mix of large mouse and small rats. he has always been a very good eater till recently. His temperature hasn't change and his layout in his tank is also the same. What I have noticed is that he is drinking a lot and being very lethargic but when hes awake he is active. He hasn't lost any weight which is a good sign but I am very worried about him as this has never happen before. I have been in touch with my local reptile vets but they couldn't pinpoint what the problem could be and suggested that I should just carry on trying to feed him as normal.
I hope you can help and give some advise on what I should do.
Thank you Kirstie.

Answer
Did you take a fresh fecal sample in, when you took him to the vet?  Internal parasites are probably the most common cause of poor appetite in colubrid snakes.  If you feed live prey (and even if you don't...though the risk is reduced with frozen/thawed), internal parasites can occasionally be transmitted.  Since he's got a record at the vet, you should be able to just take a fecal sample in to them in a baggy, without bringing the snake back in.  Just talk to the vet about it.

That would absolutely be my first guess.