Help! My cat is really skinny!?

Okay so recently my cat has become skinny. This is Ozzie's(my cats) Story: He was farel and was brought into a shelter. He then was socialized and is a love bug. He has a resputory diesese and so i clean his boogers and stuff. Recently he has became skinny. When he lies down, and you pet him on his side you can feel his ribs. We feed him a lot. He LOVES human food. I thought maybe his fav human food would fatten him up. I tried it and he didnt eat it. So then i opened another can of cat food,(we give my other cat mimi and ozzie the same food) and gave it to him. He didnt eat it. Now im wondering if there is something else we can do. AND no we cant take him to the vet as we are poor/(renting a place) PLEASE HELP!!