Everyone is going to tell you to go to the vet. If its rapidly losing weight and looks emaciated then something is wrong and only a good herp vet can actually tell you what is going on and what needs to be done. And if it doesn't want to eat give it more than 3 days between feeding attempts. If you're worried that a full week may be too long to wait then the snake is likely extremely ill.
When you say you are letting him now stay alone, what was his living conditions before? Was he not alone and not mostly free from disturbance? Healthy pythons are supposed to be alone and mostly free from disturbance to keep them healthy. When those minimums aren't met problems arise.
I am sure others will bring up temp and humidity. How are those?