Corn Snake Health Problem Help Me Please?

There was regurgitation almost every meal, sometimes one mouse would be digested, the other one would come up, sometimes both, gradually worsening since August. I found blood in his feces, was told it was probably nematode infection (worms). Got him tested by a vet, he came back negative for parasites, salmonella, and two other things, the names of which I forget. We don't know what's wrong with him, but he's very skinny now (prominent spine, sagging skin), and I'm trying to fatten him up by giving him one mouse every week rather than 2 mice every month. He's not regurgitating these anymore, but there is still some blood in his feces. I have had him for a little over a year, his feeding schedule never changed (2 frozen thawed adult mice, once a month), he's always been healthy until now. He's an adult, around 4 or 5 years old. Can anyone help me? The vet doesn't even know. Since his drastic weight loss I've also started adding calcium supplement to his water, along with electrolytes and vitamin d3.