My cat has lost a dramatic amount of weight and popped ALL OVER himself.

I noticed after coming home from a trip (both my cats had food) my cat was hiding a lot but that's usual for him. And I went to pet him ( he's a long hair cat ) and I could feel all his bones. I thought he had gotten depressed and stopped eating when we were gone because he's a very shy cat (He was at petsmart his whole life ) and today I came home and my room smelt HORRIBLE. So I found him under my bed and when he came out there was poop ALL OVER him everywhere except his face. I know i should get him to a vet but I don't have the money right now and before I spend thousands of dollars at the vet I need to know what this could be and if this is serious and putting him at risk.