I would suggest a Vet. visit.
From the evidence you've presented I would say thiaminase would be my first suspect. Not knowing the full history on the snake makes it difficult but also sheds a little light on what might be happening. I'm guessing the guy before the guy you got it from was feeding minnows also.
I think the only thing you can do at this time is switch to a more balanced diet and add in some supp.
I don't mean to be a downer but usually when a snake reaches this point because of thiaminase it is too late to do anything. I hope that isn't the case here.
Please keep us posted on this situation.