It is so scary when they loose weight, my guy lost 5lbs in about 1 month and started pooping blood, he had IDB. Inflamed Bowel, so tests would show anything (our vet never did any to save us some $$) but putting him on different food as got his weight up.
We put him on Carlyles Just Tuna for cats (80c a tin from Global Pet Foods) and he devoured it, and it didnt upset his tum / he could digest it, so he started to et his weight up.
It could be some sort of food intolerance, it took a long time for him to eventually poop blood but he had been steadily loosing weight for a while. Turns out his body has become allergic to something in his food
It was so sad to see him to bony, but now hes up to 11.5lbs!