I am sorry to hear Leo is so sick, and only 2 years old. One concern if he is heaving, losing weight, and not eating is that he could have a blockage in his intestines. This is not something you could treat at home; he would need surgery to survive a blockage.
Another concern in young cats is the feline leukemia virus.
It is hard to tell you how to treat him, if we don't know what is wrong with him. All you can do is try to get him to take liquids: tuna juice, water, broth, milk, to keep him from dehydrating more.
There are places that may help with your vet bills; these websites list them. He really needs medical attention from your description of him:
I hope you can get him the help he needs. Please let me know what else I can answer, and how he is doing.